const source = {};
for (let i = 0; i <= 10000; i++) {
const prop = `${i <= 1000 ? 'take' : 'dontTake'}_prop`;
source[prop] = i;
}
let result = Object.fromEntries(
Object
.entries(source)
.filter(([key]) => key.startsWith('take'))
)
let result = {};
for (let prop in source) {
if (prop.startsWith('take')) {
result[prop] = source[prop];
}
}