Why was the name let chosen for block-scoped variable declarations in JavaScript

Question

I understand why var takes that name - it is variable  const - it is a constant  but what is the meaning behind the name for let  which scopes to the current block  Let it be

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It does exactly what the var does with a scope difference  Now it can not take the name var since that is already taken    So it looks that it has taken the next best name which has a semantic in an interesting English language construct   let myPet    dog     In English it says  Let my pet be a dog

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It could also mean something like  Lexical Environment Type or Tied    It bothers me that it would simply be  let this be that   And let rec wouldn t make sense in lambda calculus

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Let uses a more immediate block level limited scope whereas var is function scope or global scope typically   It seems let was chosen most likely because it is found in so many other languages to define variables  such as BASIC  and many others

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The most likely possibility is that it was the most idiomatic choice  Not only is it easy to speak  but rather intuitive to understand  Some could argue  even more so than var  But I reckon there s a little more history to this  From Wikipedia   Dana Scott s LCF language was a stage in the evolution of lambda calculus into modern functional languages  This language introduced the let expression  which has appeared in most functional languages since that time  State-full imperative languages such as ALGOL and Pascal essentially implement a let expression  to implement restricted scope of functions  in block structures   I would like to believe this was an inspiration too  for the let in Javascript

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Adding to exebook s response  the mathematics usage of the keyword let also encapsulates well the scoping implications of let when used in Javascript ES6  Specifically  just as the following ES6 code is not aware of the assignment in braces of toPrint when it prints out the value of  Hello World     let toPrint    Hello World          let toPrint    Goodbye World      console log toPrint      Prints  Hello World    let as used in formalized mathematics  especially the writing of proofs  indicates that the current instance of a variable exists only for the scope of that logical idea   In the following example  x immediately gains a new identity upon entering the new idea  usually these are concepts necessary to prove the main idea  and reverts immediately to the old x upon the conclusion of the sub-proof  Of course  just as in coding  this is considered somewhat confusing and so is usually avoided by choosing a different name for the other variable   Let x be so and so      ensp  Proof stuff   ensp New Idea   Let x be something else     prove something   Conclude New Idea   ensp Prove main idea with old x

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I think JavaScript s indebtedness to Scheme is obvious here  Scheme not only has let  but has let   let -values  let-syntax  and let-values   See  The Scheme Programming Language  4th Ed       The choice adds further credence to the notion that JavaScript is Lispy  but--before we get carried away--not homoiconic

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Let is a mathematical statement that was adopted by early programming languages like Scheme and Basic  Variables are considered low level entities not suitable for higher levels of abstraction  thus the desire of many language designers to introduce similar but more powerful concepts like in Clojure  F   Scala  where let might mean a value  or a variable that can be assigned  but not changed  which in turn lets the compiler catch more programming errors and optimize code better   JavaScript has had var from the beginning  so they just needed another keyword  and just borrowed from dozens of other languages that use let already as a traditional keyword as close to var as possible  although in JavaScript let creates block scope local variable instead

User · Answer

I guess it follows mathematical tradition  In mathematics  it is often said  let x be arbitrary real number  or like that

[javascript] Why was the name 'let' chosen for block-scoped variable declarations in JavaScript?

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