Do Expressions and Optional Chaining

September 1, 2017
javascript babel

Do Expressions and Optional Chaining are two extensions that I am particularly excited about in the ECMA pipeline.

[update] I’ve removed the optional chaining syntax plugin as it is required by the transform anyway.

Do Expressions

Do Expressions solve a case when you want to declare a variable and assign it a value with a more complex expression than a single ternary. Consider the following render method which renders a color component based on props.

render() {
  const { color } = this.props;

  let component = null;
  if(color === 'blue') { component = <BlueComponent/>; }
  if(color === 'red') { component = <RedComponent/>; }
  if(color === 'green') { component = <GreenComponent/>; }

  return <div>{component}</div>
}

There are two deficencies in this code. One is that we have to declare value with let instead of const which signals that it could be modified anywhere in the rest of the code before we render component.

The second deficiency is related to the first, in that the if statements aren’t grouped except by convention. We could easily put the if statements anywhere in the render function (perhaps after a large block of other complicated code) which makes it harder to debug.

The do expression version solves these issues. We get to declare a const for component, indicating that the value won’t change later, and we also get to group the logic for defining the component value. This makes debugging the value of component easier later when we’re searching for all of the places it could be modified.

render() {
  const { color } = this.props;

  const component = do {
    if(color === 'blue') { <BlueComponent/>; }
    if(color === 'red') { <RedComponent/>; }
    if(color === 'green') { <GreenComponent/>; }
  }

  return <div>{component}</div>
}

Optional Chaining

Expanding on the previous example, let’s say we’re using GraphQL to fetch our data which is one case where deep object access can occur. The GraphQL response is passed in as data, but attempting to destructure further is risky if there are nullable values in the chain.

render() {
  const { color } =
    this.props.data &&
    this.props.data.viewer &&
    this.props.data.viewer.preferences;

  const component = do {
    if(color === 'blue') { <BlueComponent/>; }
    if(color === 'red') { <RedComponent/>; }
    if(color === 'green') { <GreenComponent/>; }
  }

  return <div>{component}</div>
}

We can clean this up into something very similar to normal prop access:

render() {
  const color = this.props.data?.viewer?.preferences?.color;

  const component = do {
    if(color === 'blue') { <BlueComponent/>; }
    if(color === 'red') { <RedComponent/>; }
    if(color === 'green') { <GreenComponent/>; }
  }

  return <div>{component}</div>
}

Note that you can’t destructure on undefined so the following can throw:

const { color } = this.props.data?.viewer?.preferences;

Babel

You’ll need a babel config like this to run the above examples, and a babel version of >= 7 which you can currently install as babel-cli@next, etc.

{
  "presets": [
    ["env", {
      "targets": {
        "node": "current"
      }
    }],
    "stage-0",
    "react"
  ],
  "plugins": [
    "transform-do-expressions",
    "transform-optional-chaining"
  ]
}

Also note that these plugins will be in stage-0, but it’s still good practice to list them out if you’re using them.

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