Intersection of two strings in JavaScript

Let's assume that we are working with the following strings and we want to know if they can merge:

javascript
let s1 = 'Hello';
let s2 = '__ll_';
let s3 = 'A____';

Cases:

1. s1 and s2 can merge (both have equal length and coincidence in chars)
2. s1 and s3 can't merge ('H' and 'A' aren't equal).

The goal is to create a function returning true for the first case and false for the second one.

javascript
const stringsCanMerge = (stringA, stringB) => {
    
    // Declaring the function, we need to compare char to char
    let res = [...stringA].reduce( (acc, char, index) => {
        
        // Getting lengths of both strings
        let lengthA = stringA.length;
        let lengthB = stringB.length;
        
        // Getting current character
        let charA = stringA[index];
        let charB = stringB[index];
        
        // Conditions:
        // 1. String should have equal length
        // 2. Characters should be equal or one of two should be a '_'
        acc = acc && lengthA === lengthB && (charA === charB || charA === '_' || charB === '_');
        
        // Returning accumulator
        return acc;
        
    }, true);
        
    return res;
    
}

You could also translate the function in one line of code:

javascript

const stringsCanMerge = (stringA, stringB) => [...stringA].reduce( (acc, char, i) => acc = acc && stringA.length === stringB.length && (stringA[i] === stringB[i] || stringA[i] === '_' || stringB[i] === '_'), true);

Let's test the function with some cases:

javascript

stringsCanMerge('Hello', '__ll_');
// true

stringsCanMerge('Hello', 'A____');
// false

stringsCanMerge('__ll_', 'A____');
// true