Enum for index signature parameter type in Typescript

Question

My goal is to have an object with attributes part of a typescript string enum.

type PARAM = "long-parameter" | "long-parameter2";

const PARAM = {
    param1: "long-parameter" as PARAM,
    param2: "long-parameter2" as PARAM,
}

var f = function(params:{[id:PARAM]:number}){}

f({ //must be valid
    [PARAM.param1]:1,
});

f({ //should display error
    "asdas":1
});

The problem is that var f = function(params:{[id:PARAM]:number}){} returns the error An index signature parameter type must be string or number.

Is there any way around that?

basarat · Accepted Answer · 2016-06-01 00:46:35Z

1

The key for an index signature must be a string or a number. This just because of how JavaScript works (everything is converted to a string in an object member lookup).

If you know the names of the member you should really declare them up front. This is shown below:

type PARAM = "long-parameter" | "long-parameter2";

const PARAM = {
    param1: "long-parameter" as PARAM,
    param2: "long-parameter2" as PARAM,
}

var f = function(params: { "long-parameter"?: PARAM, "long-parameter2"?: PARAM }) { }

f({ //must be valid
    [PARAM.param1]: 1,
});

f({ //should display error
    "asdas": 1
});

Tested with alm:

answered Jun 1, 2016 at 0:46

basarat

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1 Comment

Sean Vieira Over a year ago

How does the valid assignment work - 1 is not the string value long-parameter or long-parameter2 but the values for the interface are set to PARAM ... why does f({[PARAM.param1]: 1}) successfully compile?

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