Need functions from another file without putting them in global variables? An import statement brings exported values into the current JavaScript module. It answers two questions: what do you want, and where is it? In `import formatCurrency from './utils/format/currency'`, `formatCurrency` is the local name and the string is the module location. The location must be a string literal, not a variable: imports are identified before the current file runs. section 21.0 section 21.1 section 21.2
Choose the form that matches what the module exports:
- Default: `import formatCurrency from './utils/format/currency'`. A default export supplies one value; you choose its local name. section 21.2
- Named: `import { currency, number } from './utils/format'`. These names must match the exports. Rename locally with `import { number as formatter } from './utils/format'`—handy when two modules export the same name. section 21.2
- Namespace: `import * as format from './utils/format'`, then call `format.currency(1)`. You can also combine a default import with named or namespace imports. section 21.2
The meaning of a location string depends on the loader. In the Browserify and Webpack examples, `./file` and `./file.js` refer to a file relative to the importing file; omitting the extension can also allow a directory’s `index.js` to match. Do not treat those resolution details as a rule for every JavaScript environment. section 21.1
An imported name that changes
Suppose `./title_master` exports `let title = 'Java'` and a `setTitle(newTitle)` function that assigns to `title`. After `import { title, setTitle } from './title_master'`, logging `title`, calling `setTitle('Script')`, and logging again produces `Java`, then `Script`. You cannot assign to the imported `title` yourself: imports are read-only in the importing module. But this named import stays bound to the exported variable, so a change made by its module appears wherever it is imported, without a notification. section 21.3
If you need a module to run but need none of its values, use a side-effect import: `import './google_analytics'`. Its code executes before code in the importing file, even if the import appears later. Quick check: with `console.log('A')`, then `import './log_b'`, then `console.log('C')`, what prints first if `log_b` prints `B`? The order is `B`, `A`, `C`. section 21.4
