Javascript Object Lookups

Original post at toncho.dev

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We are going to see how object lookups can sometimes be used as a more maintainable alternative to if and switch statements.

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Swtich

The switch statement evaluates an expression, matching the value to a case clause, and then executes statements associated. What switch does is take input and provide an output based on the case clause.

const type = 'apple';

let fruit;

switch(type) {
  case 'apple':
    fruit = 'Apple';
    break;
  case 'pear':
    fruit = 'Pear';
    break;
  default:
    fruit = 'Unknown fruit!';
    break;
}

console.log(fruit);
// expected output: "Apple"

There are many problems with switch statement, it has non standard way to handle code blocks, the rest of JavaScript uses curly braces but switch does not. We are forced to add break statements within each case, which can lead to difficult debugging, make mistakes when writing each case clause for often forget to add this break statement, also the number of case clause increases, the performance of the object gets better than the average cost of the switch where the order of the cases matter. The object approach is a hash table lookup, and the switch has to evaluate each case until it hits a match and a break.

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If and Else

The if and else statements are similar to switch, but it evaluate a single value, if the specified condition is truthy the if statement executes otherwiese if the condition is falsy the else statements executes, when they are a lot of else if statements, something is really wrong and generally you should use something like switch as it’s more suited for the purpose and intention.

const getFruit = (type) => {
  if (type === 'apple') {
    type = 'Apple';
  } else if (type === 'pear') {
    type = 'Pear';
  } else {
    type = 'Unknown fruit!';
  }

  return type;
}

const type = 'apple';

console.log(getFruit(type));
// expected output: "Apple"

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Object Lookups

We use Objects all the time, either as constructors or literals. We are going to get values from the Object properties.

const getFruit = (type) => {
  return {
    'apple': 'Apple',
    'pear': 'Pear',
  }[type];
}

const type = 'apple';

console.log(getFruit(type));
// expected output: "Apple"

This example is mapping one value to another one. with the object lookup we provide a cleaner way to write the code, we don't have to worry about case clause or break statement and it's easier to read and quickly understand what's going on.