JavaScript LHS and RHS Lookup

lhs = rhs
lhs and rhs examples

I was reading the Scopes and Closure of You don't know JS book by Kyle Simpson, specifically this topic Compiler Speak.

There they mention the LHS and RHS lookup. I am failed to understand these two terms, can anyone help me to realize them?


LHS look-up is done when a variable appears on the left-hand side of an assignment operation, and an RHS look-up is done when a variable appears on the right-hand side of an assignment operation.

I think of it as follows : lhs lookup is a container lookup rhs lookup is a value lookup

You Don't Know JS: My learnings from Scope & Closures, In the execution phase, the Engine performs two types of variable lookups - LHS and RHS. LHS lookups occur when values are being assigned to variables, as in​  An LHS reference is the one wherein the compiler tries to access the container, or to be more specific, the address that is represented by the variable. An RHS reference is the one where the compiler tries to find the value that is held by the variable.


Don't think of it as a left hand side or right hand side assignment, think of it as storing the value into memory and retrieving it later.

For example when you type b in the chrome developer console, it starts the RHS lookup (retrieve the value of b) and if the value is not found, it throws ReferenceError.

In contrast when you type the b = 2 in chrome developer console, it starts LHS lookup, and if b is not found in the nested scope the JS compiler declares it in the global scope (depending on whether you are running code in strict mode or not).

For example take following code into consideration

function foo(a) {
    console.log( a + b)
}

foo( 2 );

Learning Javascript, LHS RHS, LHS = Variable assignment, or writing to memory. Think saving a text file to your hard drive. RHS = Variable lookup, or reading from memory. Thanks for contributing an answer to Stack Overflow! Please be sure to answer the question.Provide details and share your research! But avoid …. Asking for help, clarification, or responding to other answers.


I think of it as follows : lhs lookup is a container lookup rhs lookup is a value lookup

I like Kyle Simpson's approach a lot but this particular nutshell explanation brought home the point loud and clear to me.

There's always a trade between "just tell what I need to know, nothing more" and drilling down to understand better at a deep level.

Sometimes that deep understanding can help you keep out of trouble, how to debug, write test code, optimize and refactor.

I am currently reading and watching a lot of Kyle's writing and online teaching, he does have knack for explaining things well. A lot of instructors lose people along the way either by going too fast because the have expertise and it's hard for them to slow down - on the other hand if you get too basic the talk gets uninteresting and you just tune out.

I am looking for a good examples of left/right hand side of an , I just read You Don't Know Javascript: Scope and Closures chapter one. The LHS​/RHS references did not totally sink in. An RHS look-up is indistinguishable, for our purposes, from simply a look-up of the value of some  When the variable is on right hand side or in other words when engine is looking for value of the variable, it's called RHS assignment. In LHS assignment Javascript engine will be helpful and create a variable even if it's not defined earlier BUT in RHS assignment it would throu a reference undefined errror.


Simple example from the same book you have mentioned

function foo(a) {
console.log( a ); // 2
}
foo( 2 );

LHS : When you pass a value(2) to the foo method , compiler will assign to the parameter as a = 2 , is called LHS Lookup. It will simply find a container variable to assign the value.

RHS :In order to execute console.log to print a, need an RHS reference for the value of a. It's called RHS Lookup

Another Example

function foo(a) {
var b = a;
return a + b;
}
var c = foo( 2 );

3 LHS from the above example -**

  1. c = (container to hold the return of foo method)
  2. a = 2 (When you pass value 2 to the method, compiler will assign a = 2)
  3. b =

4 RHS from the above code snippet

  1. foo(2) - need a reference of a to get the value
  2. = a - to get the value of b , need a reference of a
  3. a - to get the value of a , need a reference of a
  4. b - to get the value of b , need a reference of b

Edit:

LHS & RHS Look Ups - DEV Community ‍ ‍ , view raw lhs-rhs-1.js hosted with ❤ by GitHub. This program case is 2 . Now, let's look at a program that has both a LHS and a RHS look up. Simply put, an LHS look-up writes to memory the target’s value and an RHS look-up reads value from memory. If a value is stored in both cache and register, the generator will have to optimize by taking the value from register.


LHS - Look for an identifier for assigning purposes or for assigning a value to it.

let foo;

// It's looking for foo identifier in global scope and assign value
// So it's an LHS 
foo = 1; 

// It's also LHS as it assigning new value
foo = 2;

Now, RHS - it means when you're looking for an identifier to use it (not to assign value)

function foo() {
    alert(2);
}

// this look for an identifier called foo and 
// in global scope it's a function decoration 
// and execute it
foo();

Learn Scope and Execution Example – Deep JavaScript Foundations, Kyle walks the audience through another example of how the JavaScript compiler will Remember how that look up was essentially first come, first serve. So this is the difference between LHS and RHS which is the whole reason I had you  JavaScript Execution. The second step take by the browser’s JavaScript engine. There are two terms that you need to be familiar with as we enter the execution phase: LHS and RHS. LHS stands for left hand side, and RHS stands for right hand side. LHS references are located on the left hand side of the = assignment operator.


JavaScript LHS and RHS Lookup, JavaScript LHS and RHS Lookup. likeanswer 2019-04-23. I was reading the Scopes and Closure of You don't know JS book by Kyle Simpson, specifically this​  The function call triggers an LHS lookup for a. Why? Because passing 5 as an argument implicitly assigns value to a. Notice how the target can't be determined by positioning at first glance and must be inferred. Conversely, RHS look-ups focus on the values themselves.


Hoisting in Javascript ~ A Complete Explanation, LHS stands for left hand side, and RHS stands for right hand side. LHS references are located on the left hand side of the = assignment operator. This feature is not available right now. Please try again later.


JavaScript LHS and RHS Search - javascript 🕯️, I think of it this way: lhs search is a container search rhs search is a value search the compiler sets the parameter to a = 2 , which is called LHS Lookup. A lookup is a query expression with a left-hand side, lhs; a right-hand side, rhs; and a lookup_name that is used to produce a boolean comparison between lhs and rhs such as lhs in rhs or lhs > rhs. The notation to use a lookup in an expression is <lhs>__<lookup_name>=<rhs> .