TypeScript如何推断数组文字的元素类型？

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;
}

Answer 1

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. eg:

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.