Typescript interface - index signature inconsistency

Question

If I declare an interface in Typescript with a single named property and an index signature, my assumption is that the index signature allows me to add properties to the object without syntax errors.

interface Fruit {
    colour: string;
    [otherProperties: string]: any;
}

This works when I add the properties on declaration so the following compiles OK:

let apple: Fruit = {
    colour: 'green',
    haveToPeel: false
}

but if I extend after creation

let banana: Fruit;
banana.colour = 'yellow';
banana.haveToPeel = true;

then I get a TS error

"[ts] Property 'haveToPeel' does not exist on type 'Fruit'."

I've read around on index signatures but can't get to the bottom of how I allow my objects to be extended dynamically after creation without a TS compile error.

Martin · Accepted Answer · 2016-06-10 12:49:59Z

4

This is expected behavior.

To use the index signature you need to use the index syntax -- or use the literal assignment which you demonstrate works.

let banana: Fruit;
banana.colour = 'yellow';
banana['haveToPeel'] = true;

answered Jun 10, 2016 at 12:49

Martin

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1 Comment

HankCa Over a year ago

This is no longer true since Typescript version 2.2 (github.com/Microsoft/TypeScript/milestone/34?closed=1) released 2017.02.07- specifically github.com/Microsoft/TypeScript/issues/12596. You can now do banana.haveToPeel = true;

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