Why is searching for a value in an object by key slower than using 'for in' in js?

This is because of how the JIT compiler works.

When you start a JS script with Node, the V8 starts interpreting it, while compiling it into native machine code.

Running it in the Chrome Devtools console, I get this output :

5 "<<n"
0.167724609375ms
{txt: 5} "<<m"
2: 0.262939453125ms

NodeJS output :

5 '<<n'
1: 18.684ms
{ txt: 5 } '<<m'
2: 3.713ms

But when inverting the 2 variations :

const a = { a: { txt: 1 }, b: { txt: 2 }, c: { txt: 3 }, d: { txt: 4 }, e: { txt: 5 }, f: { txt: 6 } };


console.time('2');
for (const key in a) {
    if (a[key].txt = 5) {
        const m = a[key];
        console.log(m, '<<m')
        break;
    }
}

console.timeEnd('2');
console.time('1');
let n = a['e'].txt;
console.log(n, '<<n')
console.timeEnd('1');

Output :

{ txt: 5 } '<<m'
2: 22.017ms
5 '<<n'
1: 0.245ms

As you can see, the version that is executed first takes much more time than the second.

However, if you average it, you can see that executing the key access is much faster than the for in loop.

You have an error in your program

if (a[key].txt = 5)

You are not checking if the txt property is equal to 5. You are setting the property to 5 which means you are finished after the first execution of the loop regardless.

As you can see here, testing with JS can be really a mess.

const a = { a: { txt: 1 }, b: { txt: 2 }, c: { txt: 3 }, d: { txt: 4 }, e: { txt: 5 }, f: { txt: 6 } };

let test = function(x) {
  console.log("Test "+x+" times")
  console.time('1');
  for(let i=0;i<x;i++) {
     let n = a['e'].txt;
  }

  console.timeEnd('1');
  console.time('2');

  for(let i=0;i<x;i++) {
    for (const key in a) {
        if (a[key].txt == 5) {
            const m = a[key];
            break;
        }
    }
  } 
  console.timeEnd('2');
}

test(1)
test(100)
test(100000)
test(100000)
test(100000)
test(10000000)