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Minko Gechev

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Maintaining the angular-seed, I found out that the most common problem for developers using the project is:

 Duplicate identifier 'export='. (2300)

compilation error.

In this quick tutorial I’ll show what does this error mean and how you can fix it. In order to get better understanding of the problem we’ll discuss the differences between TypeScript and JavaScript, as well as ambient type definitions and @types.

Intro

Before getting any further we need to tell a few words about the ambient type definitions of TypeScript.

If you are familiar with the differences between TypeScript and JavaScript, you can skip this section.

TypeScript is a statically typed language which is a superset of JavaScript. Being statically typed means that once we write our program it needs to go through a phase of compilation. During this phase the compiler performs type checking in order to verify correctness to some extent. For instance:

// person-hero.ts

class Person {
  talk() {
    // ...
  }
}

class Superhero extends Person {
  fly() {
    // ...
  }
}

let bar = new Person();

bar.fly();

If we try to compile the above file, the TypeScript compiler will throw the following error:

$ tsc person-hero.ts
Property 'fly' does not exists on type 'Person'.

In contrast, if we run the following file with JavaScript:

$ node person-human.js
Uncaught TypeError: bar.fly is not a function

Although both scripts throw an error, tsc throws the error compile-time, compared to node, which throws the error runtime.

Compile-time errors are easy to handle by developers because they are based on a static code analysis performed over their code. This means that the compiler is able to notify the developers about eventual mistakes before the code reaches the users.

On the other hand, in case we use dynamic typing with JavaScript, we may not find all the errors that we have made. The code of the dynamically typed languages is harder (even impossible in most cases) for advanced static code analysis so it is quite likely to not find the possible issue in our code base even after we deploy it to production.

Ambient Type Definitions

Although TypeScript is superset of JavaScript we want to be able to use programs written with TypeScript together with such written in JavaScript. For instance, lets suppose we want to use Angular 2, a framework written in TypeScript, together with jQuery.

TypeScript compiles to JavaScript, which is great. This means that once the TypeScript code has been compiled it can communicate with code written in JavaScript but how to use jQuery within our TypeScript code? Lets take a look at the following example:

// jquery-demo.ts
let a = $('.foo');

We invoke the $ function with a selector. The code above will produce the following compile-time error:

$ tsc jquery-demo.ts
Cannot find name '$'.

This is due the fact that TypeScript is much stricter compared to JavaScript. It cannot relay on the fact that we may have included a reference to jQuery somewhere in our page. TypeScript needs to have declaration of $.

That is why the ambient type definitions appeared. In order to declare that we have a global function called $ what we can do is:

// jquery-definition.ts
declare var $: Function;

let a = $('.foo');

Now if we run:

$ tsc jquery-definition.ts

# Will output the file jquery-definition.js

Notice that declare var $: Function is not a definition of the $ function but only a declaration. In case we run node jquery-definition.js we will still get a run-time error because $ is not defined.

Why Ambient Type Definitions?

The main purpose of the ambient type definitions is that they allow text editors and IDEs to perform advanced static analysis over libraries and frameworks written in JavaScript. This is a great benefit since we are able to get compile-time errors and IntelliSense during development! It is much less likely to hit runtime errors caused by misspelling a property name or just passing an object of an incorrect type to a function.

Managing Ambient Type Definitions

TypeScript’s compiler can discover type definitions distributed with libraries you use. For instance, if you install Angular 2:

$ npm i @angular/core

The corresponding type definitions of the library will be already in node_modules/@angular/core directory. Typically most libraries written in TypeScript distribute their type definitions this way.

However, there’s huge percentage of libraries out there which do not distribute their type definitions as part of their packages. It is quite likely to use the ambient type definitions for some of the most popular libraries written in JavaScript such, as jQuery, AngularJS 1.x, React, etc.

A few years back, from DefinitelyTyped, created a CLI (Command-Line Interface) manager called tsd.

Later this tool was deprecated and replaced by the more advanced one - typings. Typings allows us to download the ambient type definitions of the libraries we use.

As part of the released of TypeScript 2, Microsoft announced a way to manage the TypeScript external type definitions with the npm registry (so we can use both npm and yarn).

By default as part of tsconfig.json’s compilerOptions you can include the lib property. It contains a list of typically used type definitions. This means that if your compilation target is ES5, and you’re building for the browser you should include: es5 and dom, if you’re using ES6 features, than you should include es6 and so on. This will automatically include definitions for ES6 features such as Map, Set, etc. and the corresponding type definitions. More about the lib property can be found here.

In order to install the ambient type definitions of jQuery using npm you can use:

$ npm i @types/jquery --save-dev

Note that you usually want to use the --save-dev flag instead of --save since most likely you don’t want the users of your npm package to install the type definitions of your third-party dependencies.

Using Ambient Type Definitions

So far so good, but now how to use the provided type definition?

In the same directory where we installed jQuery’s ambient type definitions, lets create the following file:

// jquery-demo.ts

/// <reference path="./node_modules/@types/jquery/index.d.ts"/>
let height = $('.foo').height();

Although this works, it is not quite flexible. If we change the location of the definition file or the location of any file using the definitions we need to change the value of the path attribute. tsconfig.json allows us to have further flexibility:

tsconfig.json

TypeScript defines a configuration file called tsconfig.json. In this file you can provide configuration for the TypeScript compiler. Here is a sample such file:

{
  "compilerOptions": {
    "target": "es5",
    "lib": ["dom", "es6"],
    "typeRoots": [
      "./node_modules/@types"
    ]
  },
  "exclude": [
    "node_modules",
    "dist"
  ],
  "compileOnSave": false
}

In order to hint tsc where to look for the installed external type definitions you can use the typeRoots array, part of compilerOptions of tsconfig.json.

In this file we’ve set that we want to use es5 as target language and that we want to exclude the directories node_modules and dist.

Let’s also install @types/core-js in order to have type definitions for the ES6 APIs that we’ll use:

$ npm i @types/core-js

In the end our directory structure should look like:

.
├── jquery-demo.js
├── jquery-demo.ts
├── tsconfig.json
└── node_modules
    └── @types
        ├── jquery
        │   ├── package.json
        │   │── types-metadata.json
        │   └── index.d.ts
        └── core-js
            ├── package.json
            │── types-metadata.json
            └── index.d.ts

Thanks to the tsconfig.json we can compile the jquery-demo.ts file using:

$ tsc

And…we will get all these errors:

./../.npm-packages/lib/node_modules/typescript/lib/lib.es2015.core.d.ts(17,14): error TS2300: Duplicate identifier 'PropertyKey'.
node_modules/@types/core-js/index.d.ts(21,14): error TS2300: Duplicate identifier 'PropertyKey'.
node_modules/@types/core-js/index.d.ts(85,5): error TS2687: All declarations of 'name' must have identical modifiers.
node_modules/@types/core-js/index.d.ts(145,5): error TS2403: Subsequent variable declarations must have the same type.  Variable '[Symbol.unscopables]' must be of type '{ copyWithin: boolean; entries: boolean; fill: boolean; find: boolean; findIndex: boolean; keys: ...', but here has type 'any'.
node_modules/@types/core-js/index.d.ts(262,5): error TS2687: All declarations of 'flags' must have identical modifiers.
...

This is caused by multiple versions of the same type definitions. In tsconfig.json we have es6 as part of the lib array but we also have core-js in node_modules/@types. The type definitions in core-js overlap with the ones already distributed with TypeScript. In order to fix this problem we have two options:

  • Change es6 to es5 in compilerOptions’s lib property. This way TypeScript won’t include ES6 type definitions.
  • Remove core-js from node_modules. This way TypeScript will use only its internal ES6 type definitions.

Since the ES6 type definitions which come with TypeScript are more reliable by using third-party ones, lets drop node_modules/@types/core-js.

$ rm -rf node_modules/@types/core-js

If we run tsc again, we’ll get the compiled jquery-demo.js file.

We can even remove the <reference/> tag from jquery-demo.ts and everything is still going to work! The behaviour of tsc in this case will be: “Take all the files and all the type definitions from the current directory and all of its subdirectories, except the ones declared in the exclude array”. This means that we can also manage ambient type definitions with the exclude property in tsconfig.json.

Another option of tsconfig.json that we can use is the files property. If we set it, tsc will consider only the files listed there plus all the referenced files within them.

No Namespacing of Type Definitions

You might be wondering why would we have the same type definitions in both compilerOptions’s lib and core-js? The ambient type definitions in TypeScript cannot be namespaced. Why not? Well, if we use jQuery in our project and we cannot include two different sets of ambient type definitions since we have a single global jQuery object, so only a single interface.

Recap

In recap:

  • TypeScript is statically typed language, which is great since we can catch the errors we do before the users of our software!
  • TypeScript adds type definitions through type annotations which help the compiler to perform advanced static code analysis.
  • We can take advantage of static typing with non-TypeScript library by using ambient type definitions with and install them with npm or yarn.
  • The ambient type definitions are not namespaced, they are always global!
  • Fix Duplicate * error in TypeScript by using tsconfig.json’s exclude property and add there the redundant type definitions, or remove them.
  • Using <reference/> is considered a bad practice.