if文のはなし

Transcript

1. if文のはなし
   @ticktakclock
   

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2. 自己紹介
   ● 竹中貴哉
   @ticktakclock
   ●
   React, Cordova
   ●
   javascript
   始めて
   6
   ヶ月経ちました
   ● 本業は
   Android
   

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3. if ...
   

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4. const str = 'we are javascripters';
   console.log(`length: ${str.length}`);
   if (str.length > 0) { // true
   console.log('ok');
   }
   // > length: 20
   // > ok
   

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5. const str = 'we are javascripters';
   if (str.startsWith('we')) { // true
   console.log('ok');
   }
   // > ok
   

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6. if文はtrue/falseで判定するんだ！
   

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7. const str = 'we are javascripters';
   console.log(str);
   if (str) {
   console.log('ok');
   }
   // > we are javascripters
   // > ok
   

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8. const str = 'we are javascripters';
   console.log(str);
   if (str !== null) {
   console.log('ok');
   }
   // > we are javascripters
   // > ok
   

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9. const str = 'we are javascripters';
   console.log(str);
   if (str) {
   console.log('ok');
   }
   // > we are javascripters
   // > ok
   

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10. ドキュメント
    https://developer.mozilla.org/ja/docs/Web/JavaScript/Reference/Statements/if...else#Syntax
    

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11. truthy? falsy?
    

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12. truthy
    https://developer.mozilla.org/ja/docs/Glossary/truthy
    

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13. const str = 'we are javascripters';
    console.log(str);
    if (str !== null) {
    console.log('ok');
    }
    // > we are javascripters
    // > ok
    

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14. const str = 'we are javascripters';
    console.log(str);
    if (str) {
    console.log('ok');
    }
    // > we are javascripters
    // > ok
    

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15. やってみた
    console.log(0 ? 'truthy' : 'falsy'); // falsy
    console.log(1 ? 'truthy' : 'falsy'); // truthy
    console.log(-1 ? 'truthy' : 'falsy'); // truthy
    console.log('' ? 'truthy' : 'falsy'); // falsy
    console.log('test' ? 'truthy' : 'falsy'); // truthy
    console.log(undefined ? 'truthy' : 'falsy'); // falsy
    console.log(null ? 'truthy' : 'falsy'); // falsy
    console.log('undefined' ? 'truthy' : 'falsy'); // truthy
    console.log(new Date() ? 'truthy' : 'falsy'); // truthy
    

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16. まとめ
    if文の条件式は何でも入る
    

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17. string
    https://www.ecma-international.org/publications/files/ECMA-ST-ARCH/ECMA-262,%201st%20edition,%
    20June%201997.pdf
    

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18. おしまい
    

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19. string
    https://developer.mozilla.org/ja/docs/Web/JavaScript/Data_structures#Strings
    

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20. var str = '';
    console.log(str[0]); // undefined
    console.log(str.length); // 0
    console.log(str.charCodeAt(0)); // NaN
    str = '\0';
    console.log(str[0]); // ’ ’
    console.log(str.length); // 1
    console.log(str.charCodeAt(0)); // 0
    

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