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if文のはなし
@ticktakclock
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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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if ...
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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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const str = 'we are javascripters';
if (str.startsWith('we')) { // true
console.log('ok');
}
// > ok
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if文はtrue/falseで判定するんだ！
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const str = 'we are javascripters';
console.log(str);
if (str) {
console.log('ok');
}
// > we are javascripters
// > ok
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const str = 'we are javascripters';
console.log(str);
if (str !== null) {
console.log('ok');
}
// > we are javascripters
// > ok
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const str = 'we are javascripters';
console.log(str);
if (str) {
console.log('ok');
}
// > we are javascripters
// > ok
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ドキュメント
https://developer.mozilla.org/ja/docs/Web/JavaScript/Reference/Statements/if...else#Syntax
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truthy? falsy?
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truthy
https://developer.mozilla.org/ja/docs/Glossary/truthy
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const str = 'we are javascripters';
console.log(str);
if (str !== null) {
console.log('ok');
}
// > we are javascripters
// > ok
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const str = 'we are javascripters';
console.log(str);
if (str) {
console.log('ok');
}
// > we are javascripters
// > ok
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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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まとめ
if文の条件式は何でも入る
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string
https://www.ecma-international.org/publications/files/ECMA-ST-ARCH/ECMA-262,%201st%20edition,%
20June%201997.pdf
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おしまい
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string
https://developer.mozilla.org/ja/docs/Web/JavaScript/Data_structures#Strings
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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