Typescript: instanceof check doesn't smart cast

user_14388552 0
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I have an event listener callback like so:

function(ev: Event) {
    var userBox = id("user-box");
    var target = ev.target;
    // here
}

Now, I need to cast target to Element. Why does this compile:

function(ev: Event) {
    var userBox = id("user-box");
    var target = ev.target;
    if (target instanceof Element) {
        if (userBox.contains(target)) {
            // do something
        }
    }
}

... but this doesn't?

function(ev: Event) {
    var userBox = id("user-box");
    var target = ev.target;
    if (target !instanceof Element) {
        return;
    }
    if (userBox.contains(target)) {
        // do something
    }
}
typescript node.js return instanceof casting
1 Answers

variable instanceof XYZ is a single operation that yields a boolean value. It can be inverted by adding parenthesis: !(target instanceof Element). When you do target! instanceof Element, you are invoking the non-null assertion operator in TypeScript on target, which is a compile-time assertion.

E.g.

function(ev: Event) {
    var userBox = id("user-box");
    var target = ev.target;
    if (!(target instanceof Element)) {
        return;
    }
    if (userBox.contains(target)) {
        // do something
    }
}
user_143521610
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