Why doesn't TypeScript give a warning about implicit any when creating a variable whose type changes through multiple assignments?
// TypeScript allows this without any warnings
let variable;
variable = "string";
variable = 5;
// But it won't allow this
function print(s){
console.log(s);
}
This example showcases the inner workings of PR/11263. Through control flow analysis, TypeScript can determine the type of variable foo.
let foo;
foo = "bar";
foo = 2;
foo
//^? foo: number
In this scenario, the type of foo would be identified as number. Control flow analysis is effective in determining variable types at various references, ensuring no errors are reported even with the --noImplicitAny flag set.
An illustration involving changing the type of foo through assignments demonstrates the precision of control flow analysis.
let a: string
let b: number
let foo;
foo = "bar";
a = foo
b = foo // Type 'string' is not assignable to type 'number'
foo = 2;
a = foo // Type 'number' is not assignable to type 'string'
b = foo
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