Không ai hiểu tôi cả, có phải tôi là javascript?
- Javascript is confusing, As a web developer, whether you like it or not, you have to work with it.
- I share six tests, which are some magic cases of javascript that I have encountered in real combat.
- The general topic is
cái gì được in ra console?
. - Take 15 minutes, open the console and practice now.
Lesson 1. parseInt
1 2 3 4 5 6 | <span class="token keyword">const</span> a <span class="token operator">=</span> <span class="token function">parseInt</span> <span class="token punctuation">(</span> <span class="token string">'0.000005'</span> <span class="token punctuation">)</span> <span class="token punctuation">;</span> <span class="token keyword">const</span> b <span class="token operator">=</span> <span class="token function">parseInt</span> <span class="token punctuation">(</span> <span class="token string">'0.0000005'</span> <span class="token punctuation">)</span> <span class="token punctuation">;</span> <span class="token keyword">const</span> c <span class="token operator">=</span> <span class="token function">parseInt</span> <span class="token punctuation">(</span> <span class="token number">0.000005</span> <span class="token punctuation">)</span> <span class="token punctuation">;</span> <span class="token keyword">const</span> d <span class="token operator">=</span> <span class="token function">parseInt</span> <span class="token punctuation">(</span> <span class="token number">0.0000005</span> <span class="token punctuation">)</span> <span class="token punctuation">;</span> console <span class="token punctuation">.</span> <span class="token function">log</span> <span class="token punctuation">(</span> a <span class="token punctuation">,</span> b <span class="token punctuation">,</span> c <span class="token punctuation">,</span> d <span class="token punctuation">)</span> <span class="token punctuation">;</span> |
- Why d = 5?
Lesson 2. Math.max
1 2 3 4 5 6 | <span class="token keyword">const</span> a <span class="token operator">=</span> Math <span class="token punctuation">.</span> <span class="token function">max</span> <span class="token punctuation">(</span> <span class="token punctuation">[</span> <span class="token punctuation">]</span> <span class="token punctuation">)</span> <span class="token punctuation">;</span> <span class="token keyword">const</span> b <span class="token operator">=</span> Math <span class="token punctuation">.</span> <span class="token function">max</span> <span class="token punctuation">(</span> <span class="token punctuation">)</span> <span class="token punctuation">;</span> <span class="token keyword">const</span> c <span class="token operator">=</span> a <span class="token operator">===</span> b console <span class="token punctuation">.</span> <span class="token function">log</span> <span class="token punctuation">(</span> a <span class="token punctuation">,</span> b <span class="token punctuation">,</span> c <span class="token punctuation">)</span> <span class="token punctuation">;</span> |
Lesson 3. RegExp
1 2 3 4 | <span class="token keyword">const</span> reg <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">RegExp</span> <span class="token punctuation">(</span> <span class="token string">'true'</span> <span class="token punctuation">,</span> <span class="token string">'g'</span> <span class="token punctuation">)</span> console <span class="token punctuation">.</span> <span class="token function">log</span> <span class="token punctuation">(</span> reg <span class="token punctuation">.</span> <span class="token function">test</span> <span class="token punctuation">(</span> <span class="token string">'true'</span> <span class="token punctuation">)</span> <span class="token punctuation">)</span> <span class="token punctuation">[</span> <span class="token string">'true'</span> <span class="token punctuation">,</span> <span class="token string">'true'</span> <span class="token punctuation">,</span> <span class="token string">'true'</span> <span class="token punctuation">,</span> <span class="token string">'true'</span> <span class="token punctuation">]</span> <span class="token punctuation">.</span> <span class="token function">forEach</span> <span class="token punctuation">(</span> <span class="token parameter">str</span> <span class="token operator">=></span> console <span class="token punctuation">.</span> <span class="token function">log</span> <span class="token punctuation">(</span> reg <span class="token punctuation">.</span> <span class="token function">test</span> <span class="token punctuation">(</span> str <span class="token punctuation">)</span> <span class="token punctuation">)</span> <span class="token punctuation">)</span> <span class="token punctuation">;</span> |
- Why is it sometimes
true
and sometimesfalse
?
Lesson 4. Array.map and parseInt
1 2 3 4 5 | <span class="token keyword">const</span> a <span class="token operator">=</span> <span class="token function">parseInt</span> <span class="token punctuation">(</span> <span class="token string">'10'</span> <span class="token punctuation">)</span> <span class="token punctuation">;</span> <span class="token keyword">const</span> arrayA <span class="token operator">=</span> <span class="token punctuation">[</span> <span class="token string">'10'</span> <span class="token punctuation">,</span> <span class="token string">'10'</span> <span class="token punctuation">,</span> <span class="token string">'10'</span> <span class="token punctuation">,</span> <span class="token string">'10'</span> <span class="token punctuation">,</span> <span class="token string">'10'</span> <span class="token punctuation">]</span> <span class="token punctuation">.</span> <span class="token function">map</span> <span class="token punctuation">(</span> parseInt <span class="token punctuation">)</span> <span class="token punctuation">;</span> console <span class="token punctuation">.</span> <span class="token function">log</span> <span class="token punctuation">(</span> <span class="token punctuation">{</span> a <span class="token operator">:</span> a <span class="token punctuation">,</span> arrayA <span class="token operator">:</span> arrayA <span class="token punctuation">}</span> <span class="token punctuation">)</span> <span class="token punctuation">;</span> |
Lesson 5. Variable scope
1 2 3 4 5 6 7 8 9 | <span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token number">1</span> <span class="token punctuation">;</span> <span class="token keyword">function</span> <span class="token function">bar</span> <span class="token punctuation">(</span> <span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">if</span> <span class="token punctuation">(</span> <span class="token operator">!</span> a <span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token number">10</span> <span class="token punctuation">;</span> <span class="token punctuation">}</span> console <span class="token punctuation">.</span> <span class="token function">log</span> <span class="token punctuation">(</span> a <span class="token punctuation">)</span> <span class="token punctuation">;</span> <span class="token punctuation">}</span> <span class="token function">bar</span> <span class="token punctuation">(</span> <span class="token punctuation">)</span> <span class="token punctuation">;</span> |
- Why a = 10 ?
Lesson 6. toString() method
1 2 3 4 5 6 7 8 9 10 11 12 13 14 | <span class="token keyword">function</span> <span class="token function">tryCatch</span> <span class="token punctuation">(</span> <span class="token parameter">fn</span> <span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">try</span> <span class="token punctuation">{</span> console <span class="token punctuation">.</span> <span class="token function">log</span> <span class="token punctuation">(</span> <span class="token function">fn</span> <span class="token punctuation">(</span> <span class="token punctuation">)</span> <span class="token punctuation">)</span> <span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span> err <span class="token punctuation">)</span> <span class="token punctuation">{</span> console <span class="token punctuation">.</span> <span class="token function">log</span> <span class="token punctuation">(</span> err <span class="token punctuation">.</span> message <span class="token punctuation">)</span> <span class="token punctuation">}</span> <span class="token punctuation">}</span> <span class="token function">tryCatch</span> <span class="token punctuation">(</span> <span class="token punctuation">(</span> <span class="token punctuation">)</span> <span class="token operator">=></span> <span class="token number">2.</span> <span class="token punctuation">.</span> <span class="token function">toString</span> <span class="token punctuation">(</span> <span class="token punctuation">)</span> <span class="token punctuation">)</span> <span class="token punctuation">;</span> <span class="token function">tryCatch</span> <span class="token punctuation">(</span> <span class="token punctuation">(</span> <span class="token punctuation">)</span> <span class="token operator">=></span> <span class="token number">2</span> <span class="token punctuation">.</span> <span class="token function">toString</span> <span class="token punctuation">(</span> <span class="token punctuation">)</span> <span class="token punctuation">)</span> <span class="token punctuation">;</span> <span class="token function">tryCatch</span> <span class="token punctuation">(</span> <span class="token punctuation">(</span> <span class="token punctuation">)</span> <span class="token operator">=></span> <span class="token number">2</span> <span class="token punctuation">.</span> <span class="token function">toString</span> <span class="token punctuation">(</span> <span class="token punctuation">)</span> <span class="token punctuation">)</span> <span class="token punctuation">;</span> <span class="token function">tryCatch</span> <span class="token punctuation">(</span> <span class="token punctuation">(</span> <span class="token punctuation">)</span> <span class="token operator">=></span> <span class="token punctuation">(</span> <span class="token number">2</span> <span class="token punctuation">)</span> <span class="token punctuation">.</span> <span class="token function">toString</span> <span class="token punctuation">(</span> <span class="token punctuation">)</span> <span class="token punctuation">)</span> <span class="token punctuation">;</span> <span class="token function">tryCatch</span> <span class="token punctuation">(</span> <span class="token punctuation">(</span> <span class="token punctuation">)</span> <span class="token operator">=></span> <span class="token punctuation">[</span> <span class="token number">2</span> <span class="token punctuation">]</span> <span class="token punctuation">.</span> <span class="token function">toString</span> <span class="token punctuation">(</span> <span class="token punctuation">)</span> <span class="token punctuation">)</span> <span class="token punctuation">;</span> <span class="token function">tryCatch</span> <span class="token punctuation">(</span> <span class="token punctuation">(</span> <span class="token punctuation">)</span> <span class="token operator">=></span> <span class="token number">2.</span> <span class="token function">toString</span> <span class="token punctuation">(</span> <span class="token punctuation">)</span> <span class="token punctuation">)</span> <span class="token punctuation">;</span> |
- Why
2.toString()
is gettingError
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