tl;dr
- There's a new TypeScript flag/config called
--resolveJsonModule
that lets us import aJSON
using static imports. - With
--resolveJsonModule
enabled, we'll be able to have autocompletion from out JSON file props.
TypeScript 2.9 introduced a new --resolveJsonModule
compiler option that lets us import JSON modules from within TypeScript modules.
Importing JSON Modules via require Calls
Let's assume we have a Node application written in TypeScript, and let's say that we want to import the following JSON file:
{
"server": {
"nodePort": 8080
}
}
In Node, we can use a require
call to import this JSON file like any other CommonJS module:
const config = require("./config.json");
The JSON is automatically deserialized into a plain JavaScript object. This allows us to easily access the properties of our config object:
"use strict";
const express = require("express");
const config = require("./config.json");
const app = express();
app.listen(config.server.nodePort, () => {
console.log(`Listening on port ${config.server.nodePort} ...`);
});
So far, so good!
Importing JSON Files via Static import Declarations
Let's now say we want to use native ECMAScript modules instead of CommonJS modules. This means we'll have to convert our require
calls to static import
declarations:
// We no longer need the "use strict" directive since
// all ECMAScript modules implicitly use strict mode.
import * as express from "express";
import * as config from "./config.json";
const app = express();
app.listen(config.server.nodePort, () => {
console.log(`Listening on port ${config.server.nodePort} ...`);
});
Now, we get a type error in line 2. TypeScript doesn't let us import a JSON module out of the box, just like that. This was a conscious design decision made by the TypeScript team: pulling in large JSON files could potentially consume a lot of memory, which is why we need to opt into that feature by enabling the --resolveJsonModule
compiler flag:
Having people to consciously opt into this would imply the user understands the cost.
Let's head over to our tsconfig.json
file and enable the resolveJsonModule option there:
{
"compilerOptions": {
"target": "es2015",
"module": "commonjs",
"strict": true,
"moduleResolution": "node",
"resolveJsonModule": true
}
With --resolveJsonModule
enabled, we no longer get a type error in our TypeScript file. Even better, we now get type checking and autocompletion!
If we compile our TypeScript file with the compiler options shown above, we get the following JavaScript output:
"use strict";
Object.defineProperty(exports, "__esModule", { value: true });
const express = require("express");
const config = require("./config.json");
const app = express();
app.listen(config.server.nodePort, () => {
console.log(`Listening on port ${config.server.nodePort} ...`);
});
Notice that the output is pretty much identical to our initial require
version:
"use strict";
const express = require("express");
const config = require("./config.json");
const app = express();
app.listen(config.server.nodePort, () => {
console.log(`Listening on port ${config.server.nodePort} ...`);
});
And there you go! This is how to import JSON modules from within TypeScript modules, only one compiler option away.
Happy coding!
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