var output = expression;

for (var exp in expressoes) {
  output = output.replace(new RegExp(exp, 'g'), expressoes[exp]);
}

var output = expression;

for (var exp in expressoes) {
  output = output.split(exp).join(expressoes[exp]);
}

let output = '';

for (var char of expression) {
  output += char in expressoes ? expressoes[char] : char;
}

var output = '';

for (var i = 0, j = expression.length; i < j; i++) {
  var char = expression[i];

  output += char in expressoes ? expressoes[char] : char;
}