A type guard is a TypeScript feature that allows you to narrow the type of a variable within a conditional block of code. It is a way to tell the TypeScript compiler that an expression is of a certain type, based on some condition that you check at runtime.
function printLength(x: any) {
if (typeof x === 'string') {
console.log(x.length);
}
}
Type predicates are user-defined functions that work as type guards, where you define yourself how to narrow the type of an expression based on
some custom condition. These are just functions with a return type of argumentName is SomeType. Such functions return true
if the argument is of the specified type.
function isString(x: any): x is string {
return typeof x === 'string';
}
function printLength(x: any) {
if (isString(x)) {
console.log(x.length);
}
}
One of the advantages of using such a function is that in a conditional block where the condition is a type guard, the compiler automatically performs the appropriate casts, so explicit casting becomes unnecessary.
Type predicates provide a more precise, readable, and flexible way to check types in your code, which can lead to more robust and maintainable code.
This rule raises an issue when a boolean function checking for the type of its only argument can be replaced with a type predicate.
function isSomething(x: BaseType): boolean { // Noncompliant
return (<Something>x).foo !== undefined;
}
if (isSomething(v)) {
(<Something>v).foo();
(v as Something).foo();
}
Transform the function into a type predicate, adding a return type annotation that specifies the type predicate.
function isSomething(x: BaseType): x is Something {
return (<Something>x).foo !== undefined;
}
if (isSomething(v)) {
v.foo();
}