How to do typescript function chaining?

Question

I want to use function chaining in typescript.

Consider a class

export class numbOp(){
  private n;
  constructor(int num){
    this.n = num;
  }

  public add(inc = 1){
    this.n = this.n + inc;
  }

}

How do I use it as (1)

let finalNumber = new numbOp(3);
console.log(finalNumber) // Output: 3

How do I use it as (2)

let finalNumber = new numbOp(3).add();
console.log(finalNumber) // Output: 4

How do I use it as (3)

let finalNumber = new numbOp(3).add().add();
console.log(finalNumber) // Output: 5

How do I use it as (4)

let finalNumber = new numbOp(3).add().add(2).toString();
console.log(finalNumber) // Output: "6"

Please, help me out to achieve this. Thanks in advance :)

Titian Cernicova-Dragomir · Accepted Answer · 2018-12-30 00:06:12Z

You just need to return this from the functions you want to chain

class numbOp {
    private n: number;
    constructor(num: number) {
        this.n = num;
    }

    public add(inc = 1) : this { // annotation not necessary added to address comments
        this.n = this.n + inc;
        return this;
    }
    toString() { 
        return this.n;
    }

}
let finalNumber = new numbOp(3);
console.log(finalNumber + "") // Output: 3
//How do I use it as (2)
let finalNumber2 = new numbOp(3).add();
console.log(finalNumber2 + "") // Output: 4
//How do I use it as (3)
let finalNumber3 = new numbOp(3).add().add();
console.log(finalNumber3 + "") // Output: 5
//How do I use it as (4)
let finalNumber4 = new numbOp(3).add().add(2).toString();
console.log(finalNumber4) // Output: "6"

Edit

Since the console.log part seems to have become more interesting then the chain part in the comments, I'll add the ways to ensure the output in the console is a number:

Override toString and use string coercion to get the string representation of the object
Require a terminator method be called before the display (ie don't forget to call toString when you finish the chain)
Override valueOf and use the unary + operator (this will also make you class usable in binary operations

Code for last option:

class numbOp {
    private n: number;
    constructor(num: number) {
        this.n = num;
    }

    public add(inc = 1) : this { // annotation not necessary added to address comments
        this.n = this.n + inc;
        return this;
    }
    valueOf() { 
        return this.n;
    }

}
let finalNumber2 = new numbOp(3).add();
console.log(+finalNumber2) // Output: 4
console.log(1 + (+finalNumber2)) // Output: 5
console.log(1+(finalNumber2 as any as number)) // Output: 5

console.log(finalNumber) Won't work - that will return an object, not the number (nor a string), pretty sure OP's desired output is impossible without a toString call on each
@AbhijitSrivastava Either use a terminating call after the end of each chain to return the number itself, or coerce the result to a string every time
@AbhijitSrivastava you can let the compiler infer it or use this as the return type if you want to be explicit
@AbhijitSrivastava You could override valueOf instead of toString, that way you can use the unary + operator while logging the object: console.log(+finalNumber). Using +finalNumber instead of finalNumber + "" is clearer because you are logging the numeric value of finalNumber.
@ibrahimmahrir nice idea, i'll add it to the answer as an option

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