2d:null 2e:["$","title",null,{"children":"A Note on Closure | Homer White | Observable"}] 33:T974,md`Even after we ran multiplier(3), the function twice still doubles numbers. It does not triple them! *What's going on, here?* Well, when you call the function an special environment (Haverbeke and many other JavaScript people call environments "scopes") is created for it. You could call it the *run-time* environment for the function. Like any environment the run-time environment consists of a set of name-value pairs: names and the values that are bound to them. A run-time environment starts off with the parameters for the functio, bound to the arguments given to them. Thus when you issue the call multiplier(2) the run-time environment starts wih the name factor bound to the value 2. Usually a run-time environment picks up more name-value pairs as the JavaScript engine executes successive expression in the body of the function. This time, however, the function immediately returns a value -- another function in fact, a function that will take any number it is given and return that number multipled by 2. The declaration and binding:

  let twice = multiplier(2);

guarantees that in the Global Environment the name twice is bound to this function that doubles whatever it is given. Now why, exactly, does twice know to multiply by 2? It's supposed to take whatever number it was given and multiply it by factor, but how does it know what value factor has? Well, what it does is look into the environment where it was defined. (All functions that the programmer defines in JavaScript do this; the principle is called *lexical* scoping.) The envrionment in which twice was defined was the run-time environment created when we called multiplier(2). in that environment the name factor was bound to 2, so twice knows to multiply by 2. Now when you issue the call multiplier(3) another run-time environment is created. This one ends up containing just the name factor bound to the value 3. It has no effect on the behavior of twice, because the it's a name in a different environnment, *not* the environment in which twice was defined. It's like two people living in different houses: even if both people are called 'Lester', there will be no confusion.`34:[] 31:{"id":"259f64e3e50586c0","slug":"a-note-on-closure","title":"A Note on Closure","thumbnail":"2584807ab9eb7089c3330d16613eded15c36da10228eeaeeb0ff37d1222aa00e","access_level":"public","create_time":"$D2018-02-25T22:45:45.316Z","update_time":"$D2018-02-26T00:11:11.380Z","view_time":null,"first_public_time":"$D2018-02-26T00:11:53.433Z","has_unpublished_edits":null,"fork":false,"stars":0,"starred":false,"owner":{"type":"team","id":"53c0831ffe2cb008","login":"homerhanumat","name":"Homer White","avatar_url":"https://avatars-next.observableusercontent.com/4dbf45d024a78486436988b6e0ca4bff00aa1189c66332b2d54f8a4f2baf46ec","tier":"starter_2024"},"version":56,"publish_version":56,"role":null,"authored":false,"authors":[{"type":"individual","id":"f9eb8de1a4eb7ca2","login":"homerhanumat","name":"Homer White","avatar_url":"https://avatars-next.observableusercontent.com/4dbf45d024a78486436988b6e0ca4bff00aa1189c66332b2d54f8a4f2baf46ec","tier":null}],"members":[{"type":"individual","id":"f9eb8de1a4eb7ca2","login":"homerhanumat","name":"Homer White","avatar_url":"https://avatars-next.observableusercontent.com/4dbf45d024a78486436988b6e0ca4bff00aa1189c66332b2d54f8a4f2baf46ec","tier":null,"role":"editor"}],"comments":[],"custom_thumbnail":false,"trashed":false,"lapsed":null,"subscribed":null,"forks":0,"collection":null,"type":"notebook","body":{"title":"A Note on Closure","cells":[{"id":0,"value":"md`# A Note on Closure\n\nConsider the following function:`","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":2,"value":"function multiplier(factor) {\n return number => number * factor;\n}","pinned":true,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":11,"value":"md`Let's try it out:\n

\n  let twice = multiplier(2);\n  console.log(twice(5));\n

\n`","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":13,"value":"twice = multiplier(2)","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":49,"value":"twice(5)","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":20,"value":"md`Let's try it out again:\n

\n  let thrice = multiplier(3);\n  console.log(thrice(5));\n

\n`","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":21,"value":"{\n let thrice = multiplier(3);\n return thrice(5);\n}","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":38,"value":"md`And let's call twice again:\n\n

\n  console.log(twice(6));\n

\n`","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":44,"value":"twice(6)","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":36,"value":"$33","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"}],"files":"$W34","stdlib":"1","titling":"auto"}} 32:"$31:body"