Defining with expressions — JavaScript fundamentals

Variables are helpful not only for storing and reusing data but also for simplifying complex calculations. Consider an example: you need to convert euros into rubles through dollars. Banks often perform this kind of conversion via an intermediate currency when shopping abroad.

First, convert 50 euros into dollars. Suppose that one euro is $1.25:

let dollarsCount = 50 * 1.25;
console.log(dollarsCount); // => 62.5

Last lesson, we assigned a specific value to a variable. Here, when we declare our variable, let dollarsCount = 50 * 1.25; we have an expression to the right of the equals sign. The interpreter computes the result - 62.5, and records it in a variable. An interpreter doesn't care about what is in front of it: 62.5 or 50 * 1.25, both are expressions to compute. And they result in the same value, 62.5.

Any string is an expression. The concatenation of strings is also an expression. The interpreter processes an encountered expression and produces, as a result, the value of the expression. Here are some sample expressions with comments on the right containing the resulting value.

62.5             // 62.5
50 * 1.25        // 62.5
120 / 10 * 2     // 24
'hello'          // "hello"
'Good' + 'will'  // "Goodwill"

The code structure rules (language grammar) ensure that any calculation (not just math, but also things like string concatenation) can take place when an expression is expected, and the program will continue. For this reason, it's impossible to describe and show all use cases of all operations.

Programs consist of numerous combinations of expressions and understanding this concept is a major step on your journey.

Based on the above, consider whether this code will work?

let who = "dragon's" + 'mother';
console.log(who);

Now, back to our currency program. We'll record the dollar value in rubles as a separate variable. Calculate the value of 50 euros in dollars by multiplying them by 1.25. Suppose that 1 dollar is 60 rubles:

let rublesPerDollar = 60;
let dollarsCount = 50 * 1.25; // 62.5
let rublesCount = dollarsCount * rublesPerDollar; // 3750 rubles

console.log(rublesCount);

Now, let's merge our output with some text via concatenation:

let rublesPerDollar = 60;
let dollarsCount = 50 * 1.25; // 62.5
let rublesCount = dollarsCount * rublesPerDollar; // 3750

console.log('The price is ' + rublesCount + ' rubles');
// => The price is 3750 rubles

Any variable can be part of any expression. During the computation, the variable's name is replaced with its value.

The interpreter calculates the value of dollarsCount before using it in other expressions. When it comes to using the variable, Javascript "knows" the value it has already computed.

Variables and concatenation

To consolidate the previous topic, let's try using variables with concatenation. The syntax remains the same, we know how to concatenate (combine) two strings:

let what = 'Kings' + 'road';
console.log(what); // => 'Kingsroad'

…which means we can concatenate the string with a variable containing the string:

let first = 'Kings';
let what = first + 'road';

console.log(what); // => 'Kingsroad'

…and even concatenate two variables containing strings:

let first = 'Kings';
let last = 'road';

let what = first + last;
console.log(what); // => 'Kingsroad'