ceil

10 なら 10、 11 なら 20 になる関数を作りたい - IT戦記
Math.ceil は意地でも使いたくないという話かな。

((+x + a * .9) / a | 0) * a

とか？

$ cat test.js
f = eval('function(x,a)'+ readline());
for each(a in [100, 10, 1, .1, .01, .001])
  print([f(x,a) for each(x in [1234, 123.4, 12.34, 1.234, .1234])]);

$ echo "x + a - (x % a || a)" | js test.js
1300,200,100,100,100
1240,130,20,10,10
1234,124,13,2,1
1234,123.4,12.4,1.3000000000000003,0.2
1234,123.41000000000001,12.34,1.24,0.13
1234,123.40100000000001,12.34,1.234,0.124

$ echo "((+x + a * .9) / a | 0) * a" | js test.js
1300,200,100,0,0
1240,130,20,10,0
1234,124,13,2,1
1234,123.4,12.4,1.3,0.2
1234,123.4,12.34,1.24,0.13
1234,123.4,12.34,1.234,0.124

+

指摘に応えてテストを変えた。

$ cat test.js
f = eval('function(x,a)'+ readline());
for each(a in [100, 10, 1, .1, .01, .001])
  print([f(x,a) for each(x in [1001, 100.1, 10.01, 1.001, .0001])]);

$ echo "x + a - (x % a || a)" | js test.js
1100,200,100,100,100
1010,110,20,10,10
1001,101,11,2,1
1001.0000000000001,100.1,10.1,1.1,0.1
1001,100.10000000000001,10.01,1.01,0.01
1001,100.10000000000001,10.01,1.001,0.001

$ echo "((+x + a * .9) / a | 0) * a" | js test.js
1000,100,100,0,0
1010,100,10,10,0
1001,101,10,1,0
1001,100.10000000000001,10,1,0
1001,100.10000000000001,10.01,1,0
1001,100.10000000000001,10.01,1.0010000000000001,0.001

$ echo "((x - 1e-13 + a) / a | 0) * a" | js test.js
1100,200,100,100,100
1010,110,20,10,10
1001,101,11,2,1
1001,100.10000000000001,10.100000000000001,1.1,0.1
1001,100.10000000000001,10.01,1.01,0.01
1001,100.10000000000001,10.01,1.0010000000000001,0.001

$ echo "Math.ceil(x / a) * a" | js test.js
1100,200,100,100,100
1010,110,20,10,10
1001,101,11,2,1
1001,100.10000000000001,10.100000000000001,1.1,0.1
1001,100.10000000000001,10.01,1.01,0.01
1001,100.10000000000001,10.01,1.0010000000000001,0.001

この1e-14のブレはどうしようも無いのかな。