8000 Improved union type guards by ahejlsberg · Pull Request #1657 · microsoft/TypeScript · GitHub
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Merged
merged 4 commits into from
Jan 13, 2015
Merged

Improved union type guards #1657

merged 4 commits into from
Jan 13, 2015

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ahejlsberg
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Fixes #1587.

This PR improves handing of union types in type guards. Specifically, when a type guard of the form x instanceof C is true, the type of x is narrowed as follows:

  • If C is a subtype of the type of x, the type of x is narrowed to C.
  • Otherwise, if x is of a union type, all types that are not subtypes of C are removed from the type of x.

In the example

class Message {
    value: string;
}

function saySize(message: Message | Message[]) {
    if (message instanceof Array) {
        return message.length;  // message of type Message[] here
    }
}

the Message type is removed under the type guard because it is not a subtype of Array.

@JsonFreeman
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👍

ahejlsberg added a commit that referenced this pull request Jan 13, 2015
@ahejlsberg
Merge pull request #1657 from Microsoft/unionTypeGuards
cbecae3 
Improved union type guards
@ahejlsberg ahejlsberg merged commit cbecae3 into master Jan 13, 2015
@ahejlsberg ahejlsberg mentioned this pull request Jan 26, 2015
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@danielearwicker
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Note that (x instanceof Array) !== Array.isArray(x) in cross-window situations, though not that common these days.

@NN--- NN--- mentioned this pull request Mar 11, 2015
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@mhegazy mhegazy deleted the unionTypeGuards branch June 18, 2015 06:59
@battmanz
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If I modify the code as follows and add an else block, why does the compiler think that the type of message inside that else block is Message | Message[]? Shouldn't it only be Message?

class Message {
    value: string;
}

interface MessageHandler {
    handle(message: Message): void;
}

function saySize(message: Message | Message[], handler: MessageHandler) {
    if (message instanceof Array) {
        message.forEach(m => handler.handle(m));
    }
    else {
        handler.handle(message); // Argument of type 'Message | Message[]' is not assignable to parameter of type 'Message'.
    }
}

@danquirk
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@battmanz see #1719

@mhegazy
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mhegazy commented Sep 28, 2015

@battmanz this is tracked by #1719

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Successfully merging this pull request may close these issues.

Type Guard syntax for array
7 participants
@ahejlsberg @JsonFreeman @danielearwicker @battmanz @danquirk @mhegazy @msftclas
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