Questions tagged [typescript]
TypeScript is a typed superset of JavaScript that transpiles to plain JavaScript. It adds optional types to JavaScript. This tag is for questions specific to TypeScript. It is not used for general JavaScript questions.
230,454
questions
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answers
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Interfaces vs Types in TypeScript
What is the difference between these statements (interface vs type) in TypeScript?
interface X {
a: number
b: string
}
type X = {
a: number
b: string
};
user6101582
2127
votes
51
answers
1.7m
views
Can't bind to 'ngModel' since it isn't a known property of 'input'
I have this simple input in my component which uses [(ngModel)] :
<input type="text" [(ngModel)]="test" placeholder="foo" />
And I get the following error when I ...
- 62.2k
1923
votes
5
answers
666k
views
What is TypeScript and why would I use it in place of JavaScript? [closed]
What is the TypeScript language? What can it do that JavaScript or available libraries cannot do, that would give me reason to consider it?
- 21.5k
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votes
20
answers
2.5m
views
How to convert a string to number in TypeScript?
Given a string representation of a number, how can I convert it to number type in TypeScript?
var numberString: string = "1234";
var numberValue: number = /* what should I do with `numberString`? */;
- 24.6k
1341
votes
7
answers
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views
In TypeScript, what is the ! (exclamation mark / bang) operator when dereferencing a member?
When looking at the source code for a tslint rule, I came across the following statement:
if (node.parent!.kind === ts.SyntaxKind.ObjectLiteralExpression) {
return;
}
Notice the ! operator after ...
- 41.1k
1338
votes
52
answers
1.5m
views
Can't bind to 'formGroup' since it isn't a known property of 'form'
The situation
I am trying to make what should be a very simple form in my Angular application, but no matter what, it never works.
The Angular version
Angular 2.0.0 RC5
The error
Can't bind to '...
- 21.2k
1257
votes
30
answers
809k
views
How do you explicitly set a new property on `window` in TypeScript?
I setup global namespaces for my objects by explicitly setting a property on window.
window.MyNamespace = window.MyNamespace || {};
TypeScript underlines MyNamespace and complains that:
The ...
- 34.1k
1034
votes
40
answers
1.6m
views
Could not find a declaration file for module 'module-name'. '/path/to/module-name.js' implicitly has an 'any' type
I read how TypeScript module resolution works.
I have the following repository: @ts-stack/di.
After compiling the directory structure is as follows:
├── dist
│ ├── annotations.d.ts
│ ├── ...
- 29.7k
944
votes
37
answers
1.3m
views
Property '...' has no initializer and is not definitely assigned in the constructor
in my Angular app i have a component:
import { MakeService } from './../../services/make.service';
import { Component, OnInit } from '@angular/core';
@Component({
selector: 'app-vehicle-form',
...
- 9,921
943
votes
12
answers
993k
views
get and set in TypeScript
I'm trying to create get and set method for a property:
private _name: string;
Name() {
get:
{
return this._name;
}
set:
{
this._name = ???;
}
}
What's the ...
- 15.4k
861
votes
30
answers
979k
views
How do I dynamically assign properties to an object in TypeScript?
If I wanted to programatically assign a property to an object in Javascript, I would do it like this:
var obj = {};
obj.prop = "value";
But in TypeScript, this generates an error:
The property '...
- 141k
844
votes
21
answers
1.3m
views
What is "not assignable to parameter of type never" error in TypeScript?
Code is:
const foo = (foo: string) => {
const result = []
result.push(foo)
}
I get the following TS error:
[ts] Argument of type 'string' is not assignable to parameter of type 'never'.
What ...
- 14.9k
822
votes
9
answers
579k
views
Are strongly-typed functions as parameters possible in TypeScript?
In TypeScript, I can declare a parameter of a function as a type Function. Is there a "type-safe" way of doing this that I am missing? For example, consider this:
class Foo {
save(callback: ...
- 140k
819
votes
5
answers
436k
views
When to use JSX.Element vs ReactNode vs ReactElement?
I am currently migrating a React application to TypeScript. So far, this works pretty well, but I have a problem with the return types of my render functions, specifically in my functional components.
...
- 146k
777
votes
23
answers
982k
views
Element implicitly has an 'any' type because expression of type 'string' can't be used to index
Trying out TypeScript for a React project and I'm stuck on this error:
Element implicitly has an 'any' type because expression of type 'string' can't be used to index type '{ train_1: boolean; ...
- 8,716
776
votes
11
answers
281k
views
'unknown' vs. 'any'
TypeScript 3.0 introduces unknown type, according to their wiki:
unknown is now a reserved type name, as it is now a built-in type.
Depending on your intended use of unknown, you may want to ...
- 8,376
768
votes
10
answers
437k
views
About "*.d.ts" in TypeScript
I am curious about .d.ts declaration files. I was told by someone that .d.ts files are similar to .h header files in the C & C++ programming languages, however, the .d.ts files don't seem to work ...
user3221822
763
votes
31
answers
743k
views
How do I convert a string to enum in TypeScript?
I have defined the following enum in TypeScript:
enum Color{
Red, Green
}
Now in my function I receive color as a string. I have tried the following code:
var green= "Green";
var color : Color =...
- 60.1k
763
votes
20
answers
1.3m
views
How do I remove an array item in TypeScript?
I have an array that I've created in TypeScript and it has a property that I use as a key. If I have that key, how can I remove an item from it?
- 12.4k
750
votes
36
answers
1.5m
views
Unable to resolve dependency tree error when installing npm packages
When trying to install the npm packages using npm i command, I am getting the following exception:
I have tried reinstalling the Node.js package and setting the proxy to off using:
set HTTP_PROXY=
...
- 8,899
742
votes
15
answers
977k
views
How can I select an element in a component template?
Does anybody know how to get hold of an element defined in a component template? Polymer makes it really easy with the $ and $$.
I was just wondering how to go about it in Angular.
Take the example ...
- 8,043
741
votes
28
answers
624k
views
What is the type of the 'children' prop?
I have a very simple functional component as follows:
import * as React from 'react';
export interface AuxProps {
children: React.ReactNode
}
const aux = (props: AuxProps) => props....
- 13.7k
740
votes
28
answers
957k
views
No provider for HttpClient
After upgrading from angular 4.4 to 5.0 and after updating all HttpModule and Http to HttpClientModule I started to get this error.
I also added HttpModule again to be sure it's not due to some ...
- 7,481
739
votes
28
answers
1.2m
views
Is there a way to check for both `null` and `undefined`?
Since TypeScript is strongly-typed, simply using if () {} to check for null and undefined doesn't sound right.
Does TypeScript have any dedicated function or syntax sugar for this?
- 17.1k
739
votes
20
answers
958k
views
How to run TypeScript files from command line?
I'm having a surprisingly hard time finding an answer to this. With plain Node.JS, you can run any js file with node path/to/file.js, with CoffeeScript it's coffee hello.coffee and ES6 has babel-node ...
- 10.3k
734
votes
17
answers
428k
views
What is the syntax for Typescript arrow functions with generics?
The typescript handbook currently has nothing on arrow functions. Normal functions
can be generically typed with this syntax:
example:
function identity<T>(arg: T): T {
return arg;
}
What ...
- 9,201
731
votes
7
answers
739k
views
Possible to extend types in Typescript?
Say I have the following type:
type Event = {
name: string;
dateCreated: string;
type: string;
}
I now want to extend this type, i.e.
type UserEvent extends Event = {
UserId: string;
}
...
- 34.8k
706
votes
9
answers
785k
views
Enforcing the type of the indexed members of a Typescript object?
I would like to store a mapping of string -> string in a Typescript object, and enforce that all of the values map to strings. For example:
var stuff = {};
stuff["a"] = "foo"; ...
- 12.5k
699
votes
44
answers
971k
views
How to get names of enum entries?
I would like to iterate a TypeScript enum object and get each enumerated symbol name, for example:
enum myEnum { entry1, entry2 }
for (var entry in myEnum) {
// use entry's name here, e.g., "...
- 9,393
693
votes
13
answers
715k
views
How to define type for a function callback (as any function type, not universal any) used in a method parameter
Currently I have type definition as:
interface Param {
title: string;
callback: any;
}
I need something like:
interface Param {
title: string;
callback: function;
}
but the 2nd one ...
- 7,241
688
votes
21
answers
756k
views
access key and value of object using *ngFor
I am a bit confused about how to get the key and value of an object in angular2 while using *ngFor for iterating over the object. I know in angular 1.x there is a syntax like
ng-repeat="(key, value) ...
- 85.7k
673
votes
30
answers
853k
views
Interface type check with Typescript
This question is the direct analogon to Class type check in TypeScript
I need to find out at runtime if a variable of type any implements an interface. Here's my code:
interface A {
member: string;...
- 28.7k
670
votes
10
answers
543k
views
How to implement class constants?
In TypeScript, the const keyword cannot be used to declare class properties. Doing so causes the compiler to an error with "A class member cannot have the 'const' keyword."
I find myself in ...
- 40.3k
645
votes
29
answers
867k
views
How do I cast a JSON Object to a TypeScript class?
I read a JSON object from a remote REST server. This JSON object has all the properties of a typescript class (by design). How do I cast that received JSON object to a type var?
I don't want to ...
- 30.9k
641
votes
24
answers
842k
views
TypeScript getting error TS2304: cannot find name ' require'
I am trying to get my first TypeScript and DefinitelyTyped Node.js application up and running, and running into some errors.
I am getting the error "TS2304: Cannot find name 'require' " when I ...
- 7,409
622
votes
15
answers
456k
views
Does Typescript support the ?. operator? (And, what's it called?)
Does Typescript currently (or are there plans to) support the safe navigation operator of ?.
ie:
var thing = foo?.bar
// same as:
var thing = (foo) ? foo.bar : null;
Also, is there a more common ...
- 29.1k
610
votes
4
answers
537k
views
What is the Record type?
What does Record<K, T> mean in Typescript?
Typescript 2.1 introduced the Record type, describing it in an example:
// For every properties K of type T, transform it to U
function mapObject<K ...
- 14.6k
605
votes
18
answers
428k
views
Constructor overload in TypeScript
Has anybody done constructor overloading in TypeScript. On page 64 of the language specification (v 0.8), there are statements describing constructor overloads, but there wasn't any sample code given. ...
- 6,151
590
votes
10
answers
1.2m
views
How do I check whether an array contains a string in TypeScript?
Currently I am using Angular 2.0. I have an array as follows:
var channelArray: Array<string> = ['one', 'two', 'three'];
How can I check in TypeScript whether the channelArray contains a ...
- 8,721
589
votes
15
answers
688k
views
Type definition in object literal in TypeScript
In TypeScript classes it's possible to declare types for properties, for example:
class className {
property: string;
};
How do declare the type of a property in an object literal?
I've tried the ...
- 6,672
588
votes
9
answers
362k
views
The difference between "require(x)" and "import x"
I've just started working on a small node project that will interface with a MongoDB. However, I cannot seem to get the relevant node modules to import correctly, even though I have installed them ...
- 6,247
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votes
11
answers
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views
How to use `@ts-ignore` for a block?
The // @ts-ignore comment enables the TypeScript compiler to ignore the line below it.
How can one ignore a whole block of code with TypeScript?
- 8,709
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votes
20
answers
1.1m
views
Typescript: Type 'string | undefined' is not assignable to type 'string'
When I make any property of an interface optional, and while assigning its member to some other variable like this:
interface Person {
name?: string,
age?: string,
gender?: string,
occupation?:...
- 6,720
573
votes
16
answers
748k
views
How can I create an object based on an interface file definition in TypeScript?
I have defined an interface like this:
interface IModal {
content: string;
form: string;
href: string;
$form: JQuery;
$message: JQuery;
$modal: JQuery;
$submits: JQuery;
}...
user1679941
564
votes
10
answers
233k
views
An index signature parameter type cannot be a union type. Consider using a mapped object type instead
I'm trying to use the following pattern:
enum Option {
ONE = 'one',
TWO = 'two',
THREE = 'three'
}
interface OptionRequirement {
someBool: boolean;
someString: string;
}
interface ...
- 5,953
560
votes
11
answers
370k
views
Can't bind to 'formControl' since it isn't a known property of 'input' - Angular2 Material Autocomplete issue
I am trying to use Angular Material Autocomplete component in my Angular 2 project. I added the following to my template.
<md-input-container>
<input mdInput placeholder="Category" [...
- 23.2k
558
votes
12
answers
167k
views
@Directive vs @Component in Angular
What is the difference between @Component and @Directive in Angular?
Both of them seem to do the same task and have the same attributes.
What are the use cases and when to prefer one over another?
- 5,994
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votes
10
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549k
views
Use async await with Array.map
Given the following code:
var arr = [1,2,3,4,5];
var results: number[] = await arr.map(async (item): Promise<number> => {
await callAsynchronousOperation(item);
return item +...
- 11.2k
543
votes
31
answers
950k
views
How to suppress "error TS2533: Object is possibly 'null' or 'undefined'"?
I have a type:
type tSelectProtected = {
handleSelector?: string,
data?: tSelectDataItem[],
wrapperEle?: HTMLElement,
inputEle?: HTMLElement,
listEle?: HTMLElement,
resultEle?: ...
user1151646
542
votes
10
answers
235k
views
What is the difference between types String and string?
Does anyone know the difference between String and string in TypeScript? Am I correct in assuming that they ought to be the same?
var a: String = "test";
var b: string = "another test&...
- 24.6k