My code:

function test() {
  let value: number;

  for (let i = 0; i < 10; i++) {
    value = i;
    console.log(value);
  }

  return value;
}

test();

And got this:

Variable 'value' is used before being assigned

I found this very odd, as I had seen other similar problems that either used a callback or a Promise or some other asynchronous method, while I used just a synchronous for loop.

---------------------------------- Some update ------------------------

function test() {
  let value: number;

  for (let i = 0; i < 100; i++) {
    // a() is very expensive and with some effects
    const result = a(i)

    if(i===99) {
      value = result
    }

  }

  return value;
}

My code:

function test() {
  let value: number;

  for (let i = 0; i < 10; i++) {
    value = i;
    console.log(value);
  }

  return value;
}

test();

And got this:

Variable 'value' is used before being assigned

I found this very odd, as I had seen other similar problems that either used a callback or a Promise or some other asynchronous method, while I used just a synchronous for loop.

---------------------------------- Some update ------------------------

function test() {
  let value: number;

  for (let i = 0; i < 100; i++) {
    // a() is very expensive and with some effects
    const result = a(i)

    if(i===99) {
      value = result
    }

  }

  return value;
}

• javascript
• typescript
• for-loop
• variables
• variable-assignment

• Typescript doesn't know that your the loop's body will be executed at-least once and the value variable will indeed be initialized. That is why you get the error. Assigning any initial value will fix the error. – Yousaf Commented Nov 19, 2021 at 5:28

3 Answers 3

Use the non-null assertion operator to ensure that "its operand is non-null and non-undefined in contexts where the type checker is unable to conclude that fact."

function test() {
  let value!: number;

  for (let i = 0; i < 10; i++) {
    value = i;
    console.log(value);
  }

  return value;
}

test();

Result

TypeScript can't infer that anything in the loop body runs - it doesn't check that i starts at 0, and the condition is i < 10, and that the body will run at least once as a result. This behavior is very similar to the following:

function test() {
  let value: number;
  if (Math.random() < 0.5) {
    value = 5;
  }
  return value;
}

which produces the same error.

For TS to know that the value really definitely is defined at the end, you need to make it pletely unambiguous. Usually, the best way this is achieved is by defining and assigning to the variable once, with const, through array methods and helper functions - TS works best when reassignment is minimized.

I was facing this in typescript: Here was my solution that worked:

let myvariable: string | undefined;
if (typeof myvariable !== 'undefined') { 
    myvariable += "something appended"
} else {
    myvariable = "New beginning"
}