How to Fix the "Property does not exist on type {}" Error in TypeScript

Published Apr 18, 2020

Assigning properties to JavaScript objects is quite simple.

let obj = {}
obj.key1 = 1;
obj['key2'] = 'dog';

This would give me these values for obj:

obj: {
   key1: 1, 
   key2: 'dog'
}

The Problem

When we head over to TypeScript, running this code gives us the error below.

let obj = {}
obj.key1 = 1;
Property 'key1' does not exist on type '{}'.

How do we work with JSON objects in TypeScript?

Solution 1: The Quick Fix

In TypeScript, we can type a function by specifying the parameter types and return types.

Similarly, we need to type our objects, so that TypeScript knows what is and isn’t allowed for our keys and values.

Quick and dirty. A quick and dirty way of doing this is to assign the object to type any. This type is generally used for dynamic content of which we may not know the specific type. Essentially, we are opting out of type checking that variable.

let obj: any = {}
obj.key1 = 1;
obj['key2'] = 'dog';

But then, what’s the point of casting everything to type any just to use it? Doesn’t that defeat the purpose of using TypeScript?

Well, that’s why there’s the proper fix.

Solution 2: The Proper Fix

Consistency is key. In order to stay consistent with the TypeScript standard, we can define an interface that allows keys of type string and values of type any.

interface ExampleObject {
    [key: string]: any
}
let obj: ExampleObject = {};
obj.key1 = 1;
obj['key2'] = 'dog';

What if this interface is only used once? We can make our code a little more concise with the following:

let obj: {[k: string]: any} = {};
obj.key1 = 1;
obj['key2'] = 'dog';

Solution 3: The JavaScript Fix

Pure JavaScript. What if we don’t want to worry about types? Well, don’t use TypeScript ;)

Or you can use Object.assign().

let obj = {};
Object.assign(obj, {key1: 1});
Object.assign(obj, {key2: 'dog'});

What an exciting time to be alive.


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