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Invalid type coercion with == operator and strings #24991
Description
TypeScript Version: typescript@3.0.0-dev.20180609, 2.9.2, playround
Search Terms:
== as typeguard
number typeguard
number type coercion
Code
function test(level: number | string):number {
if (level == +level) {
const q2 = level; //number
return level;
}
return 0;
}
alert(test(5) + 1);
alert(test("5") + 1) Expected behavior:
Compiler error
Actual behavior:
The code compiles. As mentioned in the comment, typescript assumes that level inside the if block is a number; however in reality that condition passes also for number-like string ("5")
Activity
Kingwl commentedon Jun 22, 2018
seems narrow type in
narrowTypeByEqualitynot handle the Relax Equal Operatori would like to add a new type comparable similar
areTypesComparable,that handle the Relax Equal Operator and narrow type
follow the rule: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Equality_comparisons_and_sameness
Fix #24991: Weaken narrowing for == (#29840)