let Function Example: Syntax and Real Usage

This page explains the let function used in spreadsheet formulas (Excel, Google Sheets, LibreOffice Calc) and in JavaScript, with a concrete example for each. You will see the exact syntax, how it works step by step, and when it saves you from repeating long expressions.

Formula

In spreadsheet formulas, LET assigns a name to a calculation, then uses that name later in the same formula. The general syntax is:

LET(name1, value1, [name2, value2, ...], calculation)

Each name is a text label (no spaces, not a cell reference), and each value is an expression that can use earlier names. The final argument is the result expression, which can use any of the defined names.

In JavaScript, let is a keyword that declares a block-scoped variable:

let variableName = value;

The variable is only accessible inside the block (curly braces) where it is declared.

Worked Example

Using the calculator defaults: x = 5, y = 7, z = 3.

Step 1: assign name sum to x + y = 5 + 7 = 12.

Step 2: multiply sum by z = 12 × 3 = 36.

Result: 36. The calculator above returns 36 for those inputs.

Spreadsheet LET: Why It Exists

Before LET was added to Excel 365 and Google Sheets, a complex formula had to repeat the same sub‑calculation multiple times. For example, to compute a 20% discount on a price that already includes tax, you might write:

= (price*1.08) * 0.8

If you needed that discounted price again inside the same formula (say, to compare it to a threshold), you would write the whole price*1.08 part again. LET lets you write it once:

=LET(taxed, price*1.08, taxed*0.8)

This makes formulas shorter, easier to read, and faster to calculate because the sub‑expression is evaluated only once.

JavaScript let: Block Scoping Explained

In JavaScript, let was introduced in ES6 (2015) to fix problems with var. The key difference is scope:

• var is function‑scoped – it is visible throughout the whole function.
• let is block‑scoped – it is visible only inside the nearest pair of curly braces.

Example:

if (true) { let a = 10; var b = 20; }
console.log(b); // 20
console.log(a); // ReferenceError: a is not defined

This prevents accidental variable leakage and makes code more predictable.

How to Choose Between let, var, and const

In modern JavaScript, let is the default for variables that will change value. Use const for values that should not be reassigned, and avoid var unless you are supporting very old browsers.

• let – value changes, block‑scoped.
• const – value cannot be reassigned, block‑scoped.
• var – function‑scoped, hoisted, can cause bugs.

In spreadsheets, LET is always a good idea when you repeat a long expression more than once. It also helps debugging because you can name intermediate results.

Common Mistakes and Pitfalls

With spreadsheet LET:

• Using a name that is also a cell reference (like A1) – not allowed.
• Forgetting the final calculation argument – the formula returns an error.
• Using spaces in names – use underscores or periods instead.

With JavaScript let:

• Redeclaring the same let variable in the same block – throws a SyntaxError.
• Using the variable before the declaration – gives a ReferenceError (temporal dead zone).
• Assuming let is hoisted like var – it is hoisted but not initialized.

Real-World Use Cases

Spreadsheet example – calculating a weighted score:

=LET(score1, 85, score2, 90, weight1, 0.4, weight2, 0.6, score1*weight1 + score2*weight2)

This returns 88. Without LET, you would have to write the scores and weights twice.

JavaScript example – swapping two values:

let a = 1; let b = 2;
[a, b] = [b, a];

This uses destructuring with let to swap values cleanly.

Frequently Asked Questions

What is the difference between LET in Excel and let in JavaScript?

They are unrelated. Excel’s LET is a function that names intermediate calculations inside a formula. JavaScript’s let is a keyword that declares a block‑scoped variable. The name is the only similarity.

Can I use LET in older Excel versions?

No. LET is available in Excel 365, Excel 2021, and later. Google Sheets and LibreOffice Calc have similar functions (in LibreOffice it is LET as well, but syntax may differ slightly). Older versions will show a #NAME? error.

Why does my JavaScript code throw an error when I use let twice?

Because let does not allow redeclaration of the same variable in the same scope. You must use a different name or a different block. This is intentional to catch bugs.

Is let slower than var in JavaScript?

No significant performance difference in modern engines. The choice should be based on scope and readability, not speed.

Can I use LET to define a variable that is used in a chart?

No. LET only works inside a single formula. If you need a value for a chart, put it in a cell and reference that cell.

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in x | First number | - | 5 | - | -
in y | Second number | - | 7 | - | -
in z | Third number | - | 3 | - | -
out result | let result | - | (x + y) * z | 0