Strongly typed JavaScript solutions

JavaScript is a weakly typed (or dynamically typed) language, that is, the type of variables is undefined.

x = 5; // 5
x = x + 'A'; // '5A'

　In the above code, the variable x is a numeric value at first, and then a string. The type is completely determined by the current value. This is called a weak type.

　The advantage of weak typing is that it is very flexible and can write very concise code. However, for large projects, strong typing is more beneficial, as it can reduce the complexity of the system, detect type errors at compile time, and reduce the burden on programmers.

　Some people have been trying to make JavaScript a strongly typed language. Until strong typing is finally officially supported, this article introduces three solutions that are available now.

　1. TypeScript

　TypeScript is a programming language launched by Microsoft in 2012. It is a superset of JavaScript and can be compiled into JavaScript for execution. Its biggest feature is that it supports strong typing and ES6 Class.

　First, install TypeScript.

$ npm install -g typescript

　 Then, specify the type for the variable.

// greet.ts
function greet(person: string) {
  console.log("Hello, " + person);
}
greet([0, 1, 2]);

　The above is the code of the file greet.ts. The suffix ts indicates that this is TypeScript code. The parameters of the function greet are declared as strings, but when called, an array is passed in.

　Using the tsc command to compile a ts file into a js file will throw a type mismatch error.

$ tsc greeter.ts
greet.ts(5,9): error TS2345: Argument of type 'number[]'   
is not assignable to parameter of type 'string'.

2. Flowcheck

Flowcheck is a lightweight type assertion library that can check whether the variable type is correct at runtime.

　First, install Flowcheck.

$ npm install -g flowcheck

　 Then, write a script that declares the variable type.

function sum(a: number, b: number) {
  return a + b;
}
sum('hello','world')

Next, use the following command to convert the script into a normal JavaScript file.

$ browserify -t flowcheck -t [reactify --strip-types] \
input.js -o output.js

　The converted file is as follows.

var _f = require("flowcheck/assert");
function sum(a, b) {
    _f.check(arguments, _f.arguments([_f.number, _f.number]));
  return a + b;
}

　You can see that an assertion library is inserted into the code. Before each function is run, an assertion will be executed and an error will be reported if the types do not match.

$ node output.js
// throw new TypeError(message);
            ^
TypeError: 
Expected an instance of number got "hello",   
context: arguments / [number, number] / 0
Expected an instance of number got "world",  
context: arguments / [number, number] / 1

3. Flow

Flow is a type checking tool released by Facebook in 2014 to check the source code of React.

　The installation command is as follows.

$ npm install --global flow-bin

　If the installation is unsuccessful (this is the case for me), you need to compile it from the source code yourself.

　There are many uses of Flow, I will only give a few examples. The two tools introduced earlier can only check variables with declared types, while Flow can infer variable types.

// hello.js
/* @flow */
function foo(x) {
  return x*10;
}
foo("Hello, world!");

　The above is the file hello.js. The first line of the file is a comment indicating that Flow needs to be used to check the variable type.

$ flow check
hello.js:7:5,19: string
This type is incompatible with
/hello.js:4:10,13: number

　 Run the flow check command and get an error message: The expected parameter of function foo is a numeric value, but it is actually a string.

　Flow also supports type declaration of variables.

/* @flow */
function foo(x: string, y: number): string {
  return x.length * y;
}
foo("Hello", 42);

Another interesting feature is that Flow can convert type annotations into type declarations.

// annotation.js
/**
  @param {number} x
  @return {number}
 */
function square(x) {
  return x * x;
}
square(5);

　 Run the flow port command and you will get the following results.

$ flow port annotation.js
function square(x: number) : number {
   return x * x;
 }

　For more introduction to Flow, you can read "Exploring Flow, Facebook's Type Checker for JavaScript".