Inferring type of function parameters from usage

[yahiko00]

When dealing with large JavaScript codebases, it would be convenient if TypeScript could infer the type of function parameters from their usage in the body.

As a short example:

function inferenceParam(param) {
    let x = param * 3; // param is inferred to be a number as well as x
    ...
    let str = "Value of param : " + param; // Here param is implicitly casted to a string
    console.log(param.wrongProperty); // Error since param is a number and no wrongProperty exists on Number.
}

TypeScript inferred x is a number. I am just wondering if this could be possible the type inference could backtrack the type of param, also as a number?

[Jessidhia]

x is inferred as a number because, no matter what the operand is, the result will be a number (or an exception).

const x = ({ toString () { return '42' } }) * 3
console.log(typeof x, x) // "number", 126

TypeScript does not infer the type of function parameters, unless they come from a function expression's context.

[mhegazy]

Please see #15196

[mhegazy]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.