Article purpose
- Understand function declaration syntax
- Understand how to use functions in JavaScript
Content
A function in JavaScript is a block of code designed to perform a specific task.
Function declaration syntax in JavaScript
A JavaScript function defined with the keyword function, followed by the function name, followed by parentheses ().
Parentheses can include comma-separated parameter names.
Function names can contain letters, numbers, underscores, and dollar signs (the same rules as variables). Eg:
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<span class="token keyword">function</span> <span class="token function">name</span><span class="token punctuation">(</span><span class="token parameter">parameter1<span class="token punctuation">,</span> parameter2<span class="token punctuation">,</span> parameter3</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token comment">// code được viết ở đây</span> <span class="token punctuation">}</span> |
- The parameters of the function are listed inside parentheses ().
- The arguments of a function are the values the function receives when it is called.
- Inside the function, the arguments (parameters) act like local variables.
When is the function called?
The function is called:
- When an event occurs (when the user clicks a button)
- When it is called from JavaScript code.
- Auto call.
Return function
When JavaScript encounters the keyword return, function will stop executing. Functions usually compute a value and return it. Eg:
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<span class="token keyword">var</span> x <span class="token operator">=</span> <span class="token function">myFunction</span><span class="token punctuation">(</span><span class="token number">4</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// Hàm được gọi, trả về giá trị bằng x</span> <span class="token keyword">function</span> <span class="token function">myFunction</span><span class="token punctuation">(</span><span class="token parameter">a<span class="token punctuation">,</span> b</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> a <span class="token operator">*</span> b<span class="token punctuation">;</span> <span class="token comment">// hàm trả về tích của a và b</span> <span class="token punctuation">}</span> |
Why use functions?
Functions help us to reuse code to make it easier to read and maintain. You can use the function multiple times and with different parameters will return different results. Eg:
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<span class="token keyword">function</span> <span class="token function">toCelsius</span><span class="token punctuation">(</span><span class="token parameter">fahrenheit</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> <span class="token punctuation">(</span><span class="token number">5</span> <span class="token operator">/</span> <span class="token number">9</span><span class="token punctuation">)</span> <span class="token operator">*</span> <span class="token punctuation">(</span>fahrenheit <span class="token operator">-</span> <span class="token number">32</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span> document<span class="token punctuation">.</span><span class="token function">getElementById</span><span class="token punctuation">(</span><span class="token string">"demo"</span><span class="token punctuation">)</span><span class="token punctuation">.</span>innerHTML <span class="token operator">=</span> <span class="token function">toCelsius</span><span class="token punctuation">(</span><span class="token number">77</span><span class="token punctuation">)</span><span class="token punctuation">;</span> |
Results returned : function toCelsius(f) { return (5/9) \* (f-32); }