How to use Array deconstruction in Typescript with no Error?

Question

I wrote some code in Typescript:

for (const [a, b] of [['1', 2],['3', 4]]) {
  console.log(a.substr(0));
}

In javascript, it works, and outputs:

1
3

But in Typescript，that will cause an compile error near substr:

TS2339: Property 'substr' does not exist on type 'string | number'.   Property 'substr' does not exist on type 'number'.

Seems like the compiler cannot confirm the type of a, from string/number.

I think it's a bug of TypeScript. Am I wrong? Or if there is a better way to do this?

Nguyen Phong Thien · Accepted Answer · 2019-03-11 08:55:57Z

1

It's not a bug. Basically, when you write ['1', 2], Typescript will understand it as <string | number>[] instead of [string, number]. So if you want to force it to [number, string], you have to define the type

  for (const [a, b] of <[string, number][]>[['1', 2],['3', 4]]) {
    console.log(a.substr(0));
  }

answered Mar 11, 2019 at 8:55

Nguyen Phong Thien

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