How does TypeScript infer element type for array literals?

Question

In particular, why this code compiles (with --noImplicitAny)

function x() {
    const c = [];
    c.push({});
    c.indexOf({});
    return c;
}

while, this doesn't:

function x() {
    const c = [];
    c.indexOf({});
    c.push({});
    return c;
}

Both compiles with me, does it print out an error? Give us more information — Tareq El-Masri
– Tareq El-Masri, Commented Oct 14, 2018 at 16:46
Please use the Share button to get a URL to the TypeScript playground with this code in it. That way we can all reproduce it easily. — Heretic Monkey
– Heretic Monkey, Commented Oct 14, 2018 at 16:55
push helps determine the type of element that is in the array; indexOf doesn't. — Heretic Monkey
– Heretic Monkey, Commented Oct 14, 2018 at 17:03
To determine the type of the array you can do it this way const c: object[] = [] — Tareq El-Masri
– Tareq El-Masri, Commented Oct 14, 2018 at 17:07

Titian Cernicova-Dragomir · Accepted Answer · 2018-10-14 17:13:15Z

6

This is the expected behavior see this GitHub issue. The function described there is similar to yours, the question being why no error is raised with noImplicitAny turned on:

function foo() {
  const x = []
  x.push(3)
  return x
}

The type of the array is inferred to be an "evolving array" type. it then becomes number after the first x.push. foo should be returning number []. If the type of x is witnessed before control flow can determine its type, and error is produced. e.g.:

function foo() {
  const x = []
  x.push(3)
  return x;

  function f() { 
    x; // error, x is `any`.
  }
}

So in your case, indexOf witnesses the type of the array before a type can be determined and an error is raised. If indexOf is called after push the type is determined and no error is raised.

answered Oct 14, 2018 at 17:13

Titian Cernicova-Dragomir

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1 Comment

ironic Over a year ago

Bonus question: is using only 'x.push(value), x.unshift(value) or x[n] = value' (and not indexOf(), etc) to evolve the array type somehow hardcoded into the compiler or it is defined somehow in one of d.ts files?

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