curriculum/challenges/english/blocks/lecture-introduction-to-javascript/672d49959621885e9d3e672c.md
When working with JavaScript, you'll often declare variables to store data that you plan to use throughout your program.
In modern JavaScript, let and const are the preferred ways to declare variables, but they differ in how they handle value assignment and reassignment.
In this lesson, we'll explore how let and const differ in variable declaration, assignment, and reassignment.
The let keyword allows you to declare variables that can be updated or reassigned later. You can think of let as a flexible container – once you've stored a value in it, you can change that value as needed throughout your program.
Here's an example of declaring and assigning a variable with let:
let score = 10;
In this case, the variable score is declared and assigned the value 10. If you want to update the value later, you can easily do that:
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let score = 10;
console.log(score); // 10
score = 20;
console.log(score); // 20
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Now, score holds the value 20. This makes let particularly useful when you know the value of a variable will change as your program runs.
On the other hand, const is used to declare variables that are constant. Once you assign a value to a variable declared with const, you cannot reassign it.
This makes const ideal for values that you don't want to change accidentally during the execution of your program.
Here's an example of declaring and assigning a variable with const:
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const maxScore = 100;
console.log(maxScore); // 100
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Once maxScore is assigned the value 100, it cannot be changed:
maxScore = 200; // This will result in an error
Trying to reassign a value to a const variable will throw an error in your JavaScript console, as const variables are immutable once they are assigned.
You can declare a let variable without immediately assigning it a value, and you can assign it a value later:
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let age;
console.log(age); // undefined
age = 25;
console.log(age); // 25
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Variables declared with const must be assigned a value at the time of declaration. If you try to declare a const variable without assigning it a value, you will get an error:
const age; // Error: Missing initializer in const declaration
You should use let when you need to declare variables that will be reassigned later. For example, tracking a changing score or updating a value over time in your program.
Use const when you want to declare variables that should remain constant, like configuration values or settings that shouldn't be changed accidentally.
You can also use the var keyword, but it's not as recommended anymore. The var is kind of like let, except it has a wider scope, which is more likely to cause problems in your program.
What happens if you try to reassign a value to a variable declared with const?
The value will change without issue.
Think about what const stands for.
The original value will be updated, but a warning will be issued.
Think about what const stands for.
An error will be thrown because const variables cannot be reassigned.
The new value will be ignored, and the original value will stay the same.
Think about what const stands for.
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Which of the following is the correct way to assign the number 100 to a constant named maxScore?
const maxScore === 100;
Refer to the end of the lesson where this was discussed.
const maxScore = 100;
const maxScore <= 100;
Refer to the end of the lesson where this was discussed.
const maxScore == 100;
Refer to the end of the lesson where this was discussed.
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Can you declare a const variable without assigning it a value?
Yes, but you must assign a value later.
Think about the immutability of const.
No, const variables must be initialized at the time of declaration.
Yes, but you can only assign a number as the initial value.
Think about the immutability of const.
No, const variables must be declared and reassigned in the same line.
Think about the immutability of const.
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