Lazy Object Literal Initialization

Quite commonly in JavaScript code bases you would initialize object literals in the following manner:

let foo = {};
foo.bar = 123;
foo.bas = "Hello World";

As soon as you move the code to TypeScript you will start to get Errors like the following:

let foo = {};
foo.bar = 123; // Error: Property 'bar' does not exist on type '{}'
foo.bas = "Hello World"; // Error: Property 'bas' does not exist on type '{}'

This is because from the state let foo = {}, TypeScript infers the type of foo (left hand side of initializing assignment) to be the type of the right hand side {} (i.e. an object with no properties). So, it error if you try to assign to a property it doesn't know about.

Ideal Fix

The proper way to initialize an object in TypeScript is to do it in the assignment:

let foo = {
    bar: 123,
    bas: "Hello World",
};

This is also great for code review and code maintainability purposes.

The quick fix and middle ground lazy initialization patterns described below suffer from mistakenly forgetting to initialize a property.

Quick Fix

If you have a large JavaScript code base that you are migrating to TypeScript the ideal fix might not be a viable solution for you. In that case you can carefully use a type assertion to silence the compiler:

let foo = {} as any;
foo.bar = 123;
foo.bas = "Hello World";

Middle Ground

Of course using the any assertion can be very bad as it sort of defeats the safety of TypeScript. The middle ground fix is to create an interface to ensure

  • Good Docs
  • Safe assignment

This is shown below:

interface Foo {
    bar: number
    bas: string
}

let foo = {} as Foo;
foo.bar = 123;
foo.bas = "Hello World";

Here is a quick example that shows the fact that using the interface can save you:

interface Foo {
    bar: number
    bas: string
}

let foo = {} as Foo;
foo.bar = 123;
foo.bas = "Hello World";

// later in the codebase:
foo.bar = 'Hello Stranger'; // Error: You probably misspelled `bas` as `bar`, cannot assign string to number

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