In javascript all keys of an object are actually strings, and can be addressed using the same strings.

Your code is creating an array containing the values of the - possibly uninitialized - variables a, b and c, which then produces [undefined, undefined, undefined], or else whatever those variables contain.

You need to make keyArray contain strings instead, and then you can use the map() function to produce the desired result:

const pairs = { "a" : 1, "b" : 2, "c": 3 }
const keyArray = ["a", "b", "c"]
const values = keyArray.map(key => pairs[key]);
console.log(values);

const pairs = { 'a' : 1, 'b' : 2, 'c': 3 }

for(const [key , value] of Object.entries(pairs))
{
    console.log(key) // a , b, c
    console.log(value) // 1, 2, 3
}

const pairs = { 'a' : 1, 'b' : 2, 'c': 3 }
const keyArray = ['a', 'b', 'c']

for(const item of keyArray)
{
    console.log(pairs[item])
}

You need to make your items inside the array and object as strings