TypeScript Language Specification v1.5

function processValue(value: number | (() => number)) {
var x = typeof value !== "number" ? value() : value;
// Process number in x
}
the value parameter has type () => number in the first conditional expression and type number in the
second conditional expression, and the inferred type of x is number.
In the example
function f(x: string | number | boolean) {
if (typeof x === "string" || typeof x === "number") {
var y = x; // Type of y is string | number
}
else {
var z = x; // Type of z is boolean
}
}
the type of x is string | number | boolean in the left operand of the || operator, number | boolean in
the right operand of the || operator, string | number in the first branch of the if statement, and boolean
in the second branch of the if statement.
In the example
class C {
data: string | string[];
getData() {
var data = this.data;
return typeof data === "string" ? data : data.join(" ");
}
}
the type of the data variable is string in the first conditional expression and string[] in the second
conditional expression, and the inferred type of getData is string. Note that the data property must be
copied to a local variable for the type guard to have an effect.
In the example
class NamedItem {
name: string;
}
function getName(obj: Object) {
return obj instanceof NamedItem ? obj.name : "unknown";
}
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