类型断言

const el = document.querySelector('.el') as HTMLCanvasElement 

// or

const el = <HTMLCanvasElement>document.querySelector('.el')

const assertion

let obj = {
    name: 'aka' // string
}

let obj = {
    name: 'aka' // readonly 'aka' 
} as const

const断言还可以把数组断言成只读元组：

let arr = [1, 2, 3] as const

Not-null assertion

function liveDangerously(x?: number | undefined) {
  console.log(x!.toFixed());
}

assertion function

function isNumber(value: unknown): asserts value is number {
    if (typeof value !== 'number') throw new Error('Assert Error')
}

declare const value: unknown;
isNumber(value)

value.toFixed()
value.toPrecision()