TypeScript & as & Type Assertion
TypeScript & as & Type Assertion
Type Assertion (as)
That is not vanilla JavaScript, it is TypeScript. As any means consider the typed object as a plain untyped javascript object.
The as keyword is a Type Assertion in TypeScript which tells the compiler to consider the object as another type than the type the compiler infers the object to be.
"use strict";
/**
*
* @author xgqfrms
* @license MIT
* @copyright xgqfrms
* @created 2020-09-11
* @modified
*
* @description TypeScript Type Assertion
* @augments
* @example
* @link
*
*/
const log = console.log;
/* <input type="text" data-uid="html-input-type-assertion"> */
const dom = document.querySelector(`[data-uid="html-input-type-assertion"]`);
const inputDOM1: HTMLInputElement = dom as HTMLInputElement;
// const inputDOM1: HTMLInputElement = document.querySelector(`[data-uid="html-input-type-assertion"]`) as HTMLInputElement;
inputDOM1.value;
const inputDOM2: HTMLInputElement = <HTMLInputElement>dom;
// const inputDOM2: HTMLInputElement = <HTMLInputElement>document.querySelector(`[data-uid="html-input-type-assertion"]`);
inputDOM2.value;
const input = document.querySelector(`[data-uid="html-input-type-assertion"]`);
input.value;
/*
const input: Element | null
Object is possibly 'null'.ts(2531)
*/
input?.value;
/*
any
Property 'value' does not exist on type 'Element'.ts(2339)
*/
input?.nodeValue;
// (property) Node.nodeValue: string | null | undefined
as
// TypeScript
function validateToken(token: string) {
return token;
}
const token = 'kjadj' as string | undefined;
validateToken(token);
Type Assertion
https://www.typescriptlang.org/docs/handbook/basic-types.html#type-assertions
as-syntax
let someValue: unknown = "this is a string";
let strLength: number = (someValue as string).length;
“angle-bracket” syntax:
let someValue: unknown = "this is a string";
let strLength: number = (<string>someValue).length;
demos
https://www.typescriptlang.org/docs/handbook/advanced-types.html#type-guards-and-type-assertions
https://basarat.gitbook.io/typescript/type-system/type-assertion
var foo = {};
foo.bar = 123; // Error: property 'bar' does not exist on `{}`
foo.bas = 'hello'; // Error: property 'bas' does not exist on `{}`
interface Foo {
bar: number;
bas: string;
}
var foo = {} as Foo;
foo.bar = 123;
foo.bas = 'hello';
Advanced Types
Type Guards and Differentiating Types
let pet = getSmallPet();
let fishPet = pet as Fish;
let birdPet = pet as Bird;
if (fishPet.swim) {
fishPet.swim();
} else if (birdPet.fly) {
birdPet.fly();
}
refs
https://linguinecode.com/post/how-to-solve-typescript-possibly-undefined-value
https://stackoverflow.com/questions/55781559/what-does-the-as-keyword-do
xgqfrms 2012-2020
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