[ts] Operator '===' cannot be applied to types '"aaa"' and '"bbb"'.

[mrdulin]

TypeScript Version: 2.7.0-dev.201xxxxx

Code

//This one, tsc give me an error.
const lab = 'icon-yes';
if (lab === '321') {}  //<- [ts] Operator '===' cannot be applied to types '"icon-yes"' and '"321"'.

//All of below are correct.
const lb: string = 'icon-yes';
if (lb === '321') {}

let la = 'icon-yes';  //<- this one , I just replace `const` to `let`. Why it became correct?
if (la === '321') {}

const labc = 'icon-yes';
if (labc === 'icon-yes') {}  //<- compare the error case. Do not understand.

let labcd = 'icon-yes';
if (labcd === '321') {}

Expected behavior:

const lab = 'icon-yes'; I think the variable lab should be string type by type inference.

All of these cases should be correct.

Actual behavior:

Can not figure out why.

In const lab = "icon-yes", lab can never have a value other than "icon-yes" because it is a const. So it is safe and correct to give it a string literal type.
If you explicitly write : string we will use that. If the variable is mutable it might not always have that value, so it gets the string type.

As for this case:

const labc = 'icon-yes';
if (labc === 'icon-yes') {}  //<- compare the error case. Do not understand.

It probably should be an error because the === operation can never return false, but currently we only have the tests for === that can never return true.

[fenying]

@Andy-MS See also this

let a: boolean = false;

try {

    a = true;
    // do something else.
    a = false;
}
catch (e) {

    if (true === a) { // [ts] Operator '===' cannot be applied to types 'true' and 'false'.

        // ...
    }
}

and

let a: "a" | "b" = "a";

try {

    a = "b";
    // do something else.
    a = "a";
}
catch (e) {

    if ("b" === a) { // [ts] Operator '===' cannot be applied to types '"b"' and '"a"'.

        // ...
    }
}

@fenying See #18916 and #18478.

[typescript-bot]

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