Cutting off number at each digit to construct an array in JavaScript

We need to write a JavaScript function that takes a number and returns an array of strings, where each string represents the number cut off at each digit position.

Problem

Given a number like 246, we want to create an array containing:

• First digit: "2"
• First two digits: "24"
• All digits: "246"

Example

Here's the implementation:

const num = 246;
const cutOffEach = (num = 1) => {
    const str = String(num);
    const res = [];
    let temp = '';
    for(let i = 0; i < str.length; i++){
        const el = str[i];
        temp += el;
        res.push(temp);
    };
    return res;
};
console.log(cutOffEach(num));

[ '2', '24', '246' ]

How It Works

The function converts the number to a string, then iterates through each character, building progressively longer substrings:

// Step by step for num = 246:
const cutOffEach = (num = 1) => {
    const str = String(num);  // "246"
    const res = [];
    let temp = '';
    
    // i=0: temp = "" + "2" = "2", push "2"
    // i=1: temp = "2" + "4" = "24", push "24"  
    // i=2: temp = "24" + "6" = "246", push "246"
    
    for(let i = 0; i < str.length; i++){
        temp += str[i];
        res.push(temp);
    }
    return res;
};

console.log(cutOffEach(1357));
console.log(cutOffEach(89));

[ '1', '13', '135', '1357' ]
[ '8', '89' ]

Alternative Approach Using Substring

We can also use the substring() method:

const cutOffEachAlt = (num = 1) => {
    const str = String(num);
    const res = [];
    
    for(let i = 1; i <= str.length; i++){
        res.push(str.substring(0, i));
    }
    return res;
};

console.log(cutOffEachAlt(582));

[ '5', '58', '582' ]

Conclusion

Both approaches convert the number to a string and build an array of progressively longer substrings. The first method concatenates characters, while the second uses substring extraction.