in escodegen.js [1954:1983]
UnaryExpression: function (expr, precedence, flags) {
var result, fragment, rightCharCode, leftSource, leftCharCode;
fragment = this.generateExpression(expr.argument, Precedence.Unary, E_TTT);
if (space === '') {
result = join(expr.operator, fragment);
} else {
result = [expr.operator];
if (expr.operator.length > 2) {
// delete, void, typeof
// get `typeof []`, not `typeof[]`
result = join(result, fragment);
} else {
// Prevent inserting spaces between operator and argument if it is unnecessary
// like, `!cond`
leftSource = toSourceNodeWhenNeeded(result).toString();
leftCharCode = leftSource.charCodeAt(leftSource.length - 1);
rightCharCode = fragment.toString().charCodeAt(0);
if (((leftCharCode === 0x2B /* + */ || leftCharCode === 0x2D /* - */) && leftCharCode === rightCharCode) ||
(esutils.code.isIdentifierPartES5(leftCharCode) && esutils.code.isIdentifierPartES5(rightCharCode))) {
result.push(noEmptySpace());
result.push(fragment);
} else {
result.push(fragment);
}
}
}
return parenthesize(result, Precedence.Unary, precedence);
},