someString = 'the cat looks like a cat'
anotherString = someString.replace('cat', 'dog');

someString = 'the cat looks like a cat';
anotherString = someString.replace(/cat/g, 'dog');
// anotherString now contains "the dog looks like a dog"

anotherString = someString.replace(/cat/g, 'dog');

function multiReplace(str, match, repl) {
    do {
        str = str.replace(match, repl);
    } while(str.indexOf(match) !== -1);
    return str;
}

multiReplace("the cat looks like a cat", "cat", "dog"); // "the dog looks like a dog"